The problem is actually more complicated then it seems, because people would immediately assume that the phone app responds directly to the proximity sensor, actually it is also affected by accelerometer and something else. Therefore other non-proximity related apps would greatly affect the response of the phone app. Most of these issues have been discussed before and solved or worked around.

The script I gave you is to isolate the problem. You're patient enough to perform the test yourself and confirmed that this is a hardware problem. Hardware defect is actually rarely seen, as most people like the post above probably having software issues.

The sensors could be damaged by human mishandling. They could be damaged by slight misplaced or exposing to strong magnetic field (say by the magnets in the pouch).