It's probably an issue with the phone's camera, but it could also be a problem with the app you're using to scan the QR. First, are you able to scan other QRs? Or is the problem just with a particular QR? Second, I would try a different QR scanner. If the problem is with the app you're using, a different app may work properly. Those are the things I'd do to isolate the problem. You really need to determine where, exactly, the problem is before you can devise a solution.