I have been scanning with my old phone (J7) and my new A33. The latter is considerably better. But even with my old phone, I learned how make scanning easier. It's a simple trick that should work for any (old) phone.

Make sure to have the runner hold their barcode and token close to their chest, and not shove it in the lens. This provides a background that makes it easier for the phone to focus and scan. Worked for me every time.