Really only two things to check and that's the source (TV show) or receiver. Apple's TV app doesn't really seem to have any options for audio other than downloading multichannel audio in the settings so not quite sure what audio codec they are using. Then obviously make sure your receiver isn't set to stereo only or some predefined limitation. You can also use some form of multi-stereo/channel to at least here everything out of all speakers.
Do you have another video subscription like Netflix that says if it's in 5.1 or similar then see how your receiver works with that?