My code will not scan

The seven causes, in the order they are worth checking.

Work through these

1. Check the contrast. Dark modules on a light ground, with a real difference between them.
2. Check it is not inverted. Light on dark fails on many cameras.
3. Check the quiet zone is intact — the plain margin all the way around.
4. Check the size against the distance: roughly a tenth of the reading distance.
5. Check the print quality. Ink bleed on absorbent paper closes the gaps between modules; ask for a proof.
6. Check the logo is not too large, or turn on "improve scanability" and reprint.
7. Check the surface. A code on a curve, behind glass with a reflection, or on a crumpled bag is a code the camera cannot square up.

If it scans but goes somewhere wrong

Open the code in the app and look at its destination. A dynamic code shows exactly where it points now; change it and every printed copy follows within seconds.