This has been happening for several days now- the full video plays, but it doesn't change the arena cool down time. It happens with every ad in the daily allotment.
The same thing has happened a couple times in the past, and each time the problem persisted for a few days before suddenly resolving itself. I am thinking it may be connected to the Leaderboard Events? This time the videos stopped working on day 2 or 3 of the Crescent LB. I'm on an iPad 3.
Edit: As soon as the Leaderboard Results icon disappeared from the right side of the screen, the video feature started working correctly again and reduced/removed the arena cool down time.
Since this is has been an ongoing, intermittent problem in my game, I will watch closely and report here when and if it happens again- which I am guessing will be during the next leaderboard event.
Players on another DS forum have reporting having this same issue.