I can't get the Airplay Mirroring icon on my MacBook Pro to appear when I link with my Apple TV

I can't get the Airplay Mirroring icon on my MacBook Pro to appear when I link with my Apple TV. My MacBook Pro is a mid 2012 model upgraded to Mountain Lion OS X 10.8.2. The Apple TV software is up to date. I can get my iphone 4 to Airplay over the same network but not my MacBook. I have my macBook, Apple TV & iphone on the wifi network but have also connected my modem directly to the AppleTV. I have checked Airplay Mirroring in the Displays tab. Can anybody offer a suggestion?