Admob Free has low fill rate on Android


I already add admob banner onto my Ionic 4 app. For ios, the ad working perfectly but on android, the banner wasn’t even showing. When I troubleshoot the error, the error reason since there is “No fill”. I already search the problem and the cause is simply there is no ad to show. Just for note, I already set isTesting to false. I let the android app open for a long time (couple of hours) and then the banner appear.

My question is simply why my fill rate for android app is really low while there is a tons of ad appearing on ios devices?