Few things I would suggest:

1. Test with test ad units id provided by Google https://developers.google.com/admob/android/test-ads and see if you see any ads.
2. Try enabling test device https://developers.google.com/admob/android/test-ads#enable_test_devices
3. Check the logcat and see if there's any error
4. Add the callback to see if there's any error returned https://developers.google.com/admob/android/banner#ad_events I am assuming you should see something in the fail callback which could help understand the issue futher.

override fun onAdFailedToLoad(adError : LoadAdError) {
     // Code to be executed when an ad request fails.
   }