That can be normal as TestFlight apps can be treated worse by AdMob than Simulator or Live Apps (from the App Store). However, there can be some reasons:
- Make sure you have updated AdMob with your payment details.
- Make sure that the ads you created in AdMob are banner ads.
- Check your AdMob dashboard to see the status of your ads, are they active?
- Verify you used the correct Ad Unit Id.
- Give it 24 hours, it can take time for an ad to become active in your region