Common causes, easiest first:
- The printed code is too small. Print it at least 2 cm wide.
- There's glare from a laminated surface or a light overhead — move or matt-laminate it.
- The code is damaged, smudged, or partly covered.
- The print quality is too low, or it was scaled down after printing.
Download a fresh code from the QR Codes screen and test a printed sample with two different phones before doing a large print run.