diff --git a/README.md b/README.md
index 85fb5a1..e98d84c 100644
--- a/README.md
+++ b/README.md
@@ -212,6 +212,14 @@ it is laid out along the length and turned before printing, which is what you wa
 hangs from its short edge. Set it per site and check the bitmap preview — the two look very
 different and only one will suit how you hang them.
 
+**Label stock** at the top of the badge settings is a shortcut that fills in the size, the roll
+type and the colour option. It is not itself a saved setting — the three fields it fills are what
+get stored, which is why it can appear to "revert" when the same dimensions describe two rolls.
+
+**Diagnostics.** The site card has a **Diagnostics** button showing exactly what would be sent,
+including the `brother_ql` command line, so it can be run by hand on the host. A failed test print
+shows the printer's own words rather than a summary.
+
 **If the printer cannot be reached**, sign in still completes. The kiosk falls back to its own
 browser print dialog, and the failure is shown against the site in **Admin → Sites** with the
 reason. Admins can reprint any badge from the **On site** list.
diff --git a/public/css/admin.css b/public/css/admin.css
index 1cbdab1..80f45b9 100644
--- a/public/css/admin.css
+++ b/public/css/admin.css
@@ -400,3 +400,18 @@ tr.row-bad td { background: #fdf0f2; }
   vertical-align: -2px;
   margin-right: 2px;
 }
+
+pre.raw {
+  margin: 0 0 14px;
+  padding: 12px;
+  background: var(--paper);
+  border: 1px solid var(--rule);
+  border-radius: 3px;
+  font-family: ui-monospace, Menlo, Consolas, monospace;
+  font-size: 12.5px;
+  line-height: 1.45;
+  white-space: pre-wrap;
+  word-break: break-word;
+  max-height: 320px;
+  overflow: auto;
+}
diff --git a/public/js/admin.js b/public/js/admin.js
index 08105ba..92bd151 100644
--- a/public/js/admin.js
+++ b/public/js/admin.js
@@ -689,6 +689,7 @@ async function loadSites() {
             <button class="ghost" data-preview-badge="${s.id}">Preview badge</button>
             <button class="ghost" data-bitmap="${s.id}">Bitmap preview</button>
             ${s.printer.enabled ? `<button class="ghost" data-test-print="${s.id}">Test print</button>` : ''}
+            ${s.printer.enabled ? `<button class="ghost" data-print-diag="${s.id}">Diagnostics</button>` : ''}
           </div>
         </div>
         <dl class="site-meta">
@@ -740,6 +741,17 @@ async function loadSites() {
       window.open(`/admin/api/sites/${btn.dataset.bitmap}/badge-bitmap`, '_blank')
     )
   );
+  $$('[data-print-diag]').forEach((btn) =>
+    btn.addEventListener('click', async () => {
+      const d = await api(`/sites/${btn.dataset.printDiag}/printer-diagnostics`);
+      openModal(
+        'Printer diagnostics',
+        `<pre class="raw">${esc(JSON.stringify(d, null, 2))}</pre>`,
+        null,
+        { saveLabel: 'Close', hideCancel: true }
+      );
+    })
+  );
   $$('[data-test-print]').forEach((btn) =>
     btn.addEventListener('click', async () => {
       btn.disabled = true;
@@ -748,7 +760,17 @@ async function loadSites() {
         await api(`/sites/${btn.dataset.testPrint}/test-print`, { method: 'POST' });
         toast('Sent to the printer.');
       } catch (err) {
-        toast(err.message, true);
+        // Show the printer's own words as well as the summary, because a refusal
+        // usually names the reason and the summary cannot cover every case.
+        const raw = err.payload?.raw;
+        openModal(
+          'The printer refused the job',
+          `<p>${esc(err.message)}</p>
+           ${err.payload?.command ? `<p class="hint">Command:</p><pre class="raw">${esc(err.payload.command)}</pre>` : ''}
+           ${raw ? `<p class="hint">What the printer and brother_ql said:</p><pre class="raw">${esc(raw)}</pre>` : ''}`,
+          null,
+          { saveLabel: 'Close', hideCancel: true }
+        );
       } finally {
         btn.disabled = false;
         btn.textContent = 'Test print';
@@ -764,16 +786,29 @@ async function loadSites() {
  * 62 mm is for a different printer.
  */
 const LABEL_PRESETS = [
-  { id: 'dk22205-90', label: 'Brother DK-22205 continuous, cut at 90 mm', w: 62, h: 90, photo: true },
-  { id: 'dk11202', label: 'Brother DK-11202 die-cut 62 × 100 mm', w: 62, h: 100, photo: true },
-  { id: 'dk22251-90', label: 'Brother DK-22251 black/red continuous, cut at 90 mm', w: 62, h: 90, photo: true, accent: true },
-  { id: 'dk11208', label: 'Brother DK-11208 die-cut 38 × 90 mm', w: 38, h: 90, photo: false },
-  { id: 'dk11209', label: 'Brother DK-11209 die-cut 29 × 62 mm', w: 29, h: 62, photo: false },
-  { id: 'dk11201', label: 'Brother DK-11201 die-cut 29 × 90 mm', w: 29, h: 90, photo: false },
-  { id: 'card', label: 'Card size 86 × 54 mm (not a QL-820NWB size)', w: 86, h: 54, photo: true },
-  { id: 'dymo99014', label: 'Dymo 99014 101 × 54 mm', w: 101, h: 54, photo: true },
+  { id: 'dk22205-90', label: 'Brother DK-22205 continuous, cut at 90 mm', w: 62, h: 90, photo: true, roll: '62' },
+  { id: 'dk11202', label: 'Brother DK-11202 die-cut 62 × 100 mm', w: 62, h: 100, photo: true, roll: '62' },
+  { id: 'dk22251-90', label: 'Brother DK-22251 black/red continuous, cut at 90 mm', w: 62, h: 90, photo: true, roll: '62red', accent: true },
+  { id: 'dk11208', label: 'Brother DK-11208 die-cut 38 × 90 mm', w: 38, h: 90, photo: false, roll: '62' },
+  { id: 'dk11209', label: 'Brother DK-11209 die-cut 29 × 62 mm', w: 29, h: 62, photo: false, roll: '62' },
+  { id: 'dk11201', label: 'Brother DK-11201 die-cut 29 × 90 mm', w: 29, h: 90, photo: false, roll: '62' },
+  { id: 'card', label: 'Card size 86 × 54 mm (not a QL-820NWB size)', w: 86, h: 54, photo: true, roll: '62' },
+  { id: 'dymo99014', label: 'Dymo 99014 101 × 54 mm', w: 101, h: 54, photo: true, roll: '62' },
 ];
 
+/**
+ * Which preset the saved settings correspond to. Dimensions alone are ambiguous —
+ * DK-22205 and DK-22251 are both 62 x 90 — so the roll type decides between them.
+ */
+function matchingPreset(site) {
+  return LABEL_PRESETS.find(
+    (p) =>
+      Number(site.badge.widthMm) === p.w &&
+      Number(site.badge.heightMm) === p.h &&
+      p.roll === (site.printer.label === '62red' ? '62red' : '62')
+  );
+}
+
 function openSiteModal(site) {
   openModal(
     `Edit ${site.name}`,
@@ -787,11 +822,11 @@ function openSiteModal(site) {
          <option value="">Custom size</option>
          ${LABEL_PRESETS.map(
            (p) =>
-             `<option value="${p.id}" ${
-               Number(site.badge.widthMm) === p.w && Number(site.badge.heightMm) === p.h ? 'selected' : ''
-             }>${esc(p.label)}</option>`
+             `<option value="${p.id}" ${matchingPreset(site)?.id === p.id ? 'selected' : ''}>${esc(p.label)}</option>`
          ).join('')}
        </select></label>
+     <p class="hint">A shortcut that fills in the boxes below. The size, the roll type and the
+     colour option are what actually get saved.</p>
      <div class="modal-row">
        ${field('Width (mm)', 'widthMm', site.badge.widthMm, 'number')}
        ${field('Height (mm)', 'heightMm', site.badge.heightMm, 'number')}
@@ -1083,6 +1118,14 @@ function wireBadgePreset() {
     height.value = preset.h;
     $('#badge-photo').checked = preset.photo;
     $('#badge-accent').checked = Boolean(preset.accent);
+    // The preset knows which roll it describes, so keep the two in step. Picking
+    // the DK-22251 preset while the roll stayed on "black only" is what makes the
+    // printer refuse the job.
+    const roll = $('#modal-form [name="printerLabel"]');
+    if (roll && preset.roll) {
+      roll.value = preset.roll;
+      roll.dispatchEvent(new Event('change'));
+    }
     check();
   });
 
diff --git a/src/printer.js b/src/printer.js
index f4fc639..6804c31 100644
--- a/src/printer.js
+++ b/src/printer.js
@@ -336,24 +336,26 @@ export async function printBadge(visit, site) {
   const port = Number(site.printer_port) || 9100;
   const target = `tcp://${site.printer_host}:${port}`;
 
+  const args = [
+    '--backend', 'network',
+    '--model', site.printer_model || 'QL-820NWB',
+    '--printer', target,
+    'print',
+    '--label', labelFor(site),
+    file,
+  ];
+
   try {
-    await runBrotherQl(
-      [
-        '--backend', 'network',
-        '--model', site.printer_model || 'QL-820NWB',
-        '--printer', target,
-        'print',
-        '--label', labelFor(site),
-        file,
-      ],
-      config.printing.timeoutMs
-    );
+    await runBrotherQl(args, config.printing.timeoutMs);
     note(site.id, true, `Printed to ${site.printer_host}`);
     return { ok: true, target };
   } catch (err) {
     const friendly = explainPrintError(err.message, site.printer_host);
     note(site.id, false, friendly);
-    throw new Error(friendly);
+    const wrapped = new Error(friendly);
+    wrapped.raw = err.message;
+    wrapped.command = `${config.printing.command} ${args.join(' ')}`;
+    throw wrapped;
   } finally {
     fs.rm(file, { force: true }, () => {});
   }
diff --git a/src/routes/admin.js b/src/routes/admin.js
index 08e5b40..052ca65 100644
--- a/src/routes/admin.js
+++ b/src/routes/admin.js
@@ -510,10 +510,40 @@ router.post('/sites/:id/test-print', async (req, res) => {
     const result = await printer.printBadge(printer.sampleVisit(site), site);
     res.json({ ok: true, ...result });
   } catch (err) {
-    res.status(400).json({ error: err.message });
+    // The raw output and the exact command matter when the friendly message is
+    // not enough — a printer refusing a job says why in its own words.
+    res.status(400).json({ error: err.message, raw: err.raw || null, command: err.command || null });
   }
 });
 
+/** Everything about how this site would print, for working out why it will not. */
+router.get('/sites/:id/printer-diagnostics', (req, res) => {
+  const site = db.prepare('SELECT * FROM sites WHERE id = ?').get(req.params.id);
+  if (!site) return res.status(404).json({ error: 'Not found.' });
+  res.json({
+    site: site.name,
+    printing: {
+      enabled: Boolean(site.printer_enabled),
+      host: site.printer_host,
+      port: site.printer_port,
+      model: site.printer_model,
+      rotate: site.printer_rotate,
+    },
+    rollSetting: site.printer_label,
+    labelSentToPrinter: printer.labelFor(site),
+    redRequested: Boolean(site.badge_accent),
+    redWillPrint: printer.accentWillPrintRed(site),
+    badgeMm: { width: site.badge_width_mm, height: site.badge_height_mm },
+    lastResult: printer.printerStatus(site.id),
+    command: [
+      'brother_ql --backend network',
+      `--model ${site.printer_model || 'QL-820NWB'}`,
+      `--printer tcp://${site.printer_host}:${site.printer_port || 9100}`,
+      `print --label ${printer.labelFor(site)} badge.png`,
+    ].join(' '),
+  });
+});
+
 /** Reprints a real visitor's badge on the server's printer. */
 router.post('/visits/:id/print', async (req, res) => {
   const visit = db.prepare('SELECT * FROM visits WHERE id = ?').get(req.params.id);