I've been trying to create a new channel and have been fighting with this stupid thing all week. I'll update my JSON file and even after going into Preview and Publish and changing things and refreshing the feed there, it's literally HOURS before I see any hint of changes. Very frustrating when I'm debugging and have to wait until the next day to see if it's working or not. Writing a TVML app for Apple TV was much less of a headache since I could see changes immediately!
I think I'm almost there and am waiting for one final refresh before I submit the channel, but this entire experience has been such a pain.