hi,
I have had the same problem in the iPhone 5s , when i replaced the screen, I didn't pass these two little parts shown here onto the new display assembly , and the proximity sensor didn't work afterward. One of these little plastic parts is the front camera lens, '''the other is a little bracket meant to hold the proximity sensor in place.''' You may need a heat gun to heat the glue to get these little parts of your old assembly.
see this image:
[image|470770|align=left]
http://www.dhresource.com/albu_392699793_00-1.0x0/holder-ring-lens-plastic-holder-for-iphone.jpg