Unity 5.6 - Problem with Unity Ad (not showing ads)

I have a problem with Unity Ad. Before updating Unity to 5.6, Ad was working very fine in my game. But now, I can't make it work at all. I re-imported Unity Ad extension, checked if "builtin Ads extension" and "test mode" were enabled, but nothing happened. Uninstalled Unity and return to the previous version I had (5.5.2f1), but with no results. And the console show me no errors. I didn't touch anything in my code or game itself... Anybody knows if there's any problem with Ads ? Thank you very much for your answers !!