If you're having issues with displaying ads with AdMob, follow the steps below and select the answers that best describe your situation. Make sure that you have followed the implementation guideline (Android, iOS, Unity).
Were you able to display ads with AdMob in your app previously?
Have you received any policy violations recently?
Did you changed your ad implementation recently?
Did you see any Error Log?
Do you see a message in your account saying ads have been disabled because your PIN was never verified?
Do you see ads reported in your performance reports?
Do your ads show on another device?
Did you create your Ad Unit ID recently (less than 24h)?
Is your Ad Issue related to a Mediation Ad Unit or Mediation Setup?
Have you implemented third party network SDKs and adapters?
Have you set up AdMob mediation in your AdMob interface?
Reviewing from your AdMob Reporting, which metric is showing 0?