This has also been a problem for me.
I try to refresh my screen as well as clear my cache. If that doesn't work, fill out a ticket and the folks behind the screens take care of it quickly; they're pretty awesome.
It is very frustrating when this happens because while you're sitting there watching videos you think you're getting paid and find out that's not the case. I chalk it off to the regular television setting with no reward. If we are doing anything else, it doesn't pay so it's about the same in my eyes.
Don't get discouraged, just see what you can do to fix the problem when it does occur.
Snags and snaffu's are part of the online world and there's little we can do about them except help the web owners know it's happening so they can take care of the problem.
I'm thankful there are sites like this one that allow us to have an opportunity to earn, even when the little bumps occur.
Thanks for sharing