Unity Ads not showing on Nexus 7

This is my first app and first time trying Unity Ads. I'm presently in Beta on the Google Play Store and require users to watch an ad to receive more energy to continue to play, so working ads are very important. However, I've only been able to view one ad on my Nexus 7 tablet, all other attempts shows an image in the background with a "Video Playback error" message overlay. I get this error even with the Unity test ad.

Everything I've read points to things I know are not the problem. It is NOT an issue with the network as my ancient test phone that is far inferior to the nexus 7 has the ads display correctly. Which this also rules out some setting being off. Also, I was able to actually view one ad, which worked fine.

Please say this is something I can fix! I need reliable ads and if Unity Ads can't perform I'll have to go with another option.