There’s an old glitch that I don’t think has ever been fixed. If Continue Watching and On Deck are exactly equal, then On Deck doesn’t show. Try watching a few minutes of a movie, (which won’t appear on Deck, because it’s a movie) and see if it makes a difference. That fixed it for me a few months ago. 
EDIT: It also gives you the “Content Unavailable” message for on deck if the conditions above are met.
Edit 2: On Deck - Content Unavailable
