I have tested my apps in android 7 version, its showing ads, but not in android 6.
recently i got email from google that We should update Admob SDK for receiving ads.
“Starting on 23 January 2018, we will no longer support Google Mobile Ads (GMA) SDK versions lower than 7.0.0 for Android”
see this link
https://support.google.com/admob/answer/7562314?ctx=email
but my apps stopped showing ads today