If you use ecpm floors it is fully normal that you don't get an ad every time you request one. If the user for some reason don't get any ad served (to high floor or any other reason), the app immidiately try to request one again, and again.. This can go into an infinite loop where the app make hundreds of ad requests in a very short period of time.
This will also decrease your match rate.
I believe Admob will probably block serving of ads to these users totally since the get swarmed with never ending requests.