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.
Virgin Mobile Canada - Samsung Galaxy Note 3 SM-N900W8 (CM 11 nightly) & Galaxy NOTEPRO 12.2 WiFi (stock TW ROM)