Just resolved the following issue and would like to share it :)
About 30% of our interstitials didn't get displayed on our WindowsPhone.
You sometimes read the console message "Loaded ad with sourcecode: + html code" but the code was then never loaded into the weview, meaning e.g. this log "Ad loaded into webview" never appeared.
The reason seems to be in the Admob_OnAdReceived(object sender, AdEventArgs e) callback, where we then call admobInterstitialAd.ShowAd(); automatically, as soon as the ad is loaded.
The wrong way is to cast
InterstitialAd admobInterstitialAd= (InterstitialAd)sender;
in Admob_OnAdReceived.
The right way is to create a member variable and assign it when you create the ad:
admobInterstitialAd = new InterstitialAd(this.admobAdUnitId);
Maybe that helps some people running in the same, wrong way.