So when my Apple TV was not showing on my iMAC iTunes (both were up-to-date), I started wondering "What the Heck?"
In those situations, I try to find the culprit step by step. I checked the firewall settings (since the firewall feels under attack when Apple TV and iTunes sync), Remove the blocked addresses (why someone did that?), restart the whole set (Apple TV, iMAC, iTunes). Nope.
I then decided to remove the Apple TV from the iMAC (was not showing up under device but still in Preferences), reboot the full set. Nope
I decide to disconnect the Apple TV from the iMAC (on the Apple TV side) and reconnect it. Nope.
Eventually I resolve to reste it to the factory settings. Once restored, I made sure I was on the same frequency for the Wifi (we have a two frequencies router), and, and, and, and.....Voila. It showed up again.