I believe my reward ads are working just fine. You can see it in Football Dash after you lose the 1st time, watch reward video to continue playing.
I'm sure you've checked your admobs IDs, but you may want to double check them. Are you also queueing up the reward video before you need to display it?
I just entered reward amount of 1 and called the reward item whatever I wanted. For mine, I call it, "Prevent tackle".
Are your other ads working? Just the reward that's not displaying?