iPhone 5 screen does not activate after a phone call
Sadly - you might need to take it back. The proximity sensor is a small part and if it doesn't sense that your head/ear/face has moved away from the receiver slot - the screen will not light up again.

You can save yourself a trip by backing it up (yet again) and restoring it in iTunes  - but it seems you have already ruled that out.

One thing I have seen in practice is that some people's ears have openings large enough for the proximity sensor to sense the space when the phone is pressed next to their ear as actually being away from an object.

Again, it's often best to lie the phone flat on the table and call some sort of automated help system and use a pencil eraser or your finger to test how and where your particular sensor is sensitive. Once you've spent a handful of minutes discovering how narrow the beam is and exactly how many mm the range is to engage and disengage the sensor, can you take it in for service and know the unit you have is working according to specifications.