For apps with no ads showing, first check to make sure Monetize tab is setup correctly.
1. Correct App ID
2. Admob ad type match correctly. Eg: Banner ID should not be used for Interstitial Ads
3. Appeal to Google if your admob app is block by Google. Sometimes you will receive email from Google with title "Google AdMob ad serving has been disabled to your application"
4. Make sure you don't violate any Google Admob policies
For anyone with admob ad issue, please make sure use test ad unit before you published it to app store. Using real ad unit Id in test app is against admob policy, your app could be banned by admob without your knowledge
Sample ad units
The quickest way to enable testing is to use Google-provided test ad units. These ad units are not associated with your AdMob account, so there's no risk of your account generating invalid traffic when using these ad units. Here are sample ad units that point to specific test creatives for each format: