Q: AirPlay Mirroring not working with MacBook Pro

Hi all, a bit of a mystery here.

I can stream from iTunes using AirPlay, and I can mirror using my iPhone 4s, but for some reason when I try mirroring from my MacBook Pro (mid-2012, Yosemite) I get the "Couldn't connect to Apple TV" message.

When I attempt mirroring from the MBP, however, it attempts to communicate with the ATV 3 (I can see it's state change) but after about 30 seconds of the spinning beachball, I get the could not connect message.  This happens when I try to connect via the AirPlay icon in the menu bar, and when I try to select Apple TV as the Airplay Display option in the Display window in System Preferences.  It just doesn't allow me to select the Apple TV option in the System Prefs-->Displays.  The mirroring has worked in the past, but for some reason has stopped working. 

I've read the troubleshooting articles (e.g., About AirPlay Mirroring in OS X - Apple Support) and all my other settings seem right.  The Apple TV is about 6 feet from the Airport Extreme, and I have plenty of bandwidth. The ATV has been updated and restarted, as has my modem and AE.

Can anyone think of anything I might have missed?

Cheers,

Lloyd