- try an app that is supposed to use airplay.
This happens from time to time. I'd like apple to include some kind of 'find apple TV' button. Not likely!
I use the you tube app. It is excellent at finding Apple TV.
Be sure all connections / resets are right and play a you tube video from the app. The AirPlay icon poppa up more reliably than in apple native apps ie music or photo.
Hope it helps.