While updating a 5-year-old game, I noticed the ads weren't showing consistently. Just got this error from Google: the "Smart banners" in the Mobile Advert plugin are no longer supported. Google points to using the new adaptive banners instead.
The plugin doesn't have that option, but I'm using standard and large options, and they work.
Just wanted to point this out, if anybody else has had issues with ads not showing, it might be this setting.
developers.google.com/admob/android/banner/smart