Although you read the article How to add Admob to an app? and do what it takes, however still can’t see ads, there might be some reasons like below
1. Incorrect Entry of Ad Code
Especially saving the ad code as missing or extra spaces is the reason for this. Please make sure that your ad code is entered in the correct format, with no extra spaces left or right.
2. The Ad Code Is New
If you have just created the ad codes you have added to your app, it may take some time for Admob to show ads. This period may take up to 48 hours.
3. The Problem About Admob
If your app usually shows ads but occasionally doesn’t show ads, this may be due to the lack of appropriate ads from Admob. It may not be able to fill 100% of the ad requests it receives from your app.
For instance, if an app sends 100 ad requests that have been shown 80 ads by Admob and no ads were shown 20 times, we can assume that the ad fill rate in this app is 80%. This rate may be different for each app and may change by decreasing and increasing from time to time.
4. Banned App
If your app has been banned by Admob for any reason or removed from Google Play, no ads will be shown. If Admob has stopped showing ads because of being banned, it will notify you by sending an e-mail. In addition, from the Admob control panel, you can see the reason for the suspension of the ad display in your app. You may have the chance to reactivate the ads in your app by following the instructions in the message sent to you by Admob.
After adding your ad codes, we recommend that you generate a new APK from the dashboard tab and publish it on Google Play. Thus, you will not have any loss in advertising displays. You can use Step 5: Generate Your Android File (APK) article on the subject.
If you implemented the above steps completed, however still facing the same problem, you can contact us via firstname.lastname@example.org.