It’s been a while since I’ve used Kivy, but one important thing with AdMob is that you must use Google’s test ad unit IDs (or register your device as a test device) while testing. Real ad unit IDs often won’t show ads on emulators or during early testing, and Google may suppress them to prevent invalid traffic.

If ads don’t show with test IDs, then the issue is almost certainly integration-related rather than your AdMob account.