You actually have to put in a request in your code, something like this:
AdView adView = (AdView) findViewById(R.id.adView);
AdRequest request = new AdRequest.Builder().build();
adView.loadAd(request);
Otherwise it just takes up layout space, but doesn't do anything. Not sure why they forced added code with the Google Play Services version of Admob, but they do, so...
Note that you only need to do this once, after you've added this it will auto-reload them according to your Admob settings.