I have just sorted one of these out. We replaced a full display and after a couple of weeks customer got in touch to say the double tap on function did not work and when on a phone call, the screen would not illuminate when removed from ear until the other person hung up.
Proximity sensor covers these issues.
There was an very slight gap between the rubber proximity sensor seal and the screen glass, gently pushed the rubber seal onto the glass, reinstalled the sensor and put a small bit of card over the top of the sensor before putting the black plastic cover back onto the top circuit board.
Result, sensor works great on calls and double tap works both to wake and switch screen off