AdMob ads not showing on Windows Phone 8 device

I have a problem with AdMob on Windows Phone 8. The ads show a great emulator, but not on the device itself (Nokia Lumia 520). The "FailedToReceiveAd" event is triggerd, error code: "NetworkError".

PS I have put the correct AdUnitId. I tried this on google google apps app. The result is the same.

Any ideas?

Respectfully,

Tom