Admob Banner Has Shown Only Twice in Two Weeks

I integrated Admob for iOS around a week ago. Editor gives the desired output messages. And the test ads work perfectly fine. But I have only seen banner ads for twice during this week. Whenever I try, I get "no ads to show" message. Yes, this is self explanatory. But I tried more than 100 requests during 7 days and only 2 of them became matched requests. On the other hand, I also integrated interstitials and they work perfectly fine. Am I doing something wrong with the banners? Or is it ok to have such fill rates in 7 days for banners?