AGK's AdMob implementation is glitchy and using an old SDK.
And.... I just found this!
On the site to download the SDK latest version (6.4.1 -- AppGameKit uses 6.1.0)
https://developers.google.com/mobile-ads-sdk/download#downloadandroid
Quote: "Deprecated. On August 1, 2014, Google Play will stop accepting new or updated apps that use the old standalone Google Mobile Ads SDK v6.4.1 or lower. You must upgrade to the Google Play version of the Mobile Ads SDK by then."
So it looks like AppGameKit will be required to update their implementation to use Google Play services. Hopefully we can also get them to implement Google Play services for game leaderboards and google sign in.
It is also possible that you're just not getting a fill for your ads.