It can take a few hours for ads to start being sent to the game the first time. Also, make sure you have disabled the default test ad and mopub default ads in the mopub dashboard so that only the admob ad is enabled (marked green in the dashboard).
Thanks for your answer! I have been testing for a few days, so I doubt that is the problem. I'll try disabling the test and default ads on MoPub, but even with those on shouldn't something appear?