AppleTV 4th gen won't do 1080p

I have a new AppleTV and am trying to use it with my 1080p TV. Settings show 1080p is available but it is set at 720p. When I try to change it to 1080p screen goes black for a number of seconds then I get a display saying the setting has fallen back to 720p. I don't understand because my Roku displays 1080p just fine.