Proximity sensor not working after screen replacement

Hello.

I have replaced my mom's screen on her iPhone 5c as a Mother's Day present, and both her and I are happy with the results. Anyways, for the past 4 days, my mom told me that when she's on a call, she accidentally presses button not the call menu (mute, dial pad, end button, etc). When she told me yesterday, I immediately knew that it was the proximity sensor, and I told her that was fixable. Earlier today, I opened up my mom's iPhone, and noticed the proximity sensor bracket on the original screen. I transferred that bracket to the new screen and put the iPhone back together. I called myself from her iPhone to test the sensor. The display turned off for like a second and it turned back on. It did that for like 3 times before the sensor wouldn't work anymore. Later on, like 3 hours later, I opened it up again and reseated the flex cable. I put the iPhone back together and it didn't work. 5 minutes later, I called myself again and the sensor was working! Then 20 minutes later, I called myself again to make sure that it was working again, but sadly it stopped working. I don't know what to do anymore to fix this. I kept looking online to fix this problem, and they didn't help. Right now the iPhone is functioning without the proximity sensor working.

Does anyone know a solution to fix this? It would be greatly appreciated!

Thanks