iphone 5s proximity sensor not working

I've noticed recently the proximity sensor has stopped working. I first noticed it when I tried to listen to a voice memo in iMessage and then when I kept putting my phone calls on mute and hold.

I know when I push really hard on the screen during a call the screen does go black but if I put my face directly on the screen, during a call, the phone stays lit.

I have a good case on it and a screen protector. The phone is in good condition. I've never used it without a case/screen protector.

I've tried to turn off the auto brightness, reset, but nothing is helping.