there are a few thread for proximity sensors problems... search and you will find some useful information.
but in short, there is probably some dust in front of the proximity sensor.
a temporary fix is to blow some air in the earpiece... but the problem will eventually come back.
I do this once a month.
The other thing you can do is disassemble your phone, clean the proximity sensor lens, maybe add some kind of protection to protect it from dust... and reassemble everything.
Videotron - Samsung Galaxy Note 3 SM-N900W8 (HLTE - CM 11 nightly or Temasek builds) & Galaxy NOTEPRO 12.2 WiFi (stock TW ROM)