First, check to see if the QR code is printed correctly. If the QR code is upside down, try flipping it over. If the QR code is too small, try enlarging it.
If the QR code is printed correctly and is the right size, but is still not working, try using a different QR code reader app. Some apps are better at reading QR codes than others.
If you're still having trouble, there could be something wrong with the QR code itself. If you generated the QR code, check to see if there are any errors in the URL or data. If you didn't generate the QR code, contact the person or company who did to let them know there's a problem.