Hi! Have you tried to log in using a different browser or the desktop app?
I can't watch ads using Google Chrome (updated), maybe the same situation as you. Although I have the mark of ads available, the pop up video player appears in black and any ad is playing.
My solution was to log in using Mozilla Firefox browser or YoWorld desktop app. I can watch ads and get more coins with any problem.
I hope this helps you and you can get your coins again
