I'm really a PC player, but I do use a tablet to watch vids and rack up the credits, and I frequently have this problem. I find it works fine for a while, but will suddenly, and for no apparent reason, stop awarding the credits for videos watched. Fortunately, I find it easy to resolve.
When that happens, (maybe once a week), I simply un-install the WGT app, and re-install it. It only take a minute or so, and then I'm back to getting the credits again.
Hope this helps,
Steve