Quick answer: A checkout UI extension can be built and deployed but still not appear until the app is installed on the correct store, the extension version is active, the target is supported, and the merchant adds the block in the checkout and accounts editor where required.
The real symptom
- The extension appears in local dev preview but not production checkout.
- The app is installed but no app block appears in the checkout editor.
- A reviewer cannot see the extension.
- A static target renders nowhere or only on one checkout profile.
Likely causes
- The extension was deployed but not released.
- The app block was not added in the checkout editor.
- Development Store Preview is the only place it was visible.
- The extension target does not match the surface being edited.
- The store, checkout profile, or app install is not the one being tested.
- A JavaScript error prevents the extension from rendering.
Fix path
- Confirm the app is installed on the exact store being tested.
- Deploy and release the extension version.
- Open Settings → Checkout and customize the correct checkout profile.
- Add the app block if the extension uses a block target.
- Test with the target placement the extension supports.
- Check extension logs and browser console errors.
Simple checks
- Compare dev preview, checkout editor, and live checkout separately.
- Remove and re-add the block after releasing a changed version.
- Confirm the app’s checkout targets match the extension code.
Do not do this
Do not assume deployment means merchant placement. For block extensions, the checkout editor is part of the installation path.