Been trying for a few days to cast to my first and second generation Shield TV's without much luck, I can connect but nothing plays, after contacting them they said they only support the Chromecast
Confusing as I didn't think there was a different between a Chromecast and a Cast enabled device
Edit: typos
I think the issue is the Google Cast Receiver app published by Google for Android TV devices. The All4 app also will not cast to the Shield TV properly. Rolling this app back to previous versions sems to alter (but not fix) casting from both of these apps. This has been an ongoing discussion / complaint on the All4 beta testers community on Google+. Casting from both apps to the shield worked fine about a year ago. If you look at the play store page for the Google Cast Receiver app it's nothing but complaints from Android TV owners.