im watching a movie on my phone but when i try to air play to my apple tv its

If you can AirPlay other content, then it is likely that the cause of your issue is with the website itself. Possibly the web browser tries to forward the video to the Apple TV, but despite appearing compatible to the phone software, perhaps it is not compatible.

• Is this a downloaded movie or a streaming movie? Is the source on your phone or on the internet?
• Is this an iTunes Store movie, or from a different source (e.g. Netflix)?