Some are having trouble getting APM Telemetry Radios to talk to each other and establish a link. Quite often it is a Windows driver problem. This video show how to correct this problem.
This should work for telemetry radios on the Ardupilot Mega, PixHawk, Arducopter, RTFhawk and HKPilot.

Banggood forum Win driver thread:
http://forum.banggood.com/forumtopic...

Silabs Windows Driver Download:
https://www.silabs.com/Support%20Docu...

Win driver direct download:
http://www.silabs.com/Support%20Docum...