Admob shows only 1 SAME ad (ANDROID)

I have a admob plugin in unity3d that shows ads.
Now I’ve completed every step to have ads on my game, and I DO Have ads, but I can
only see only 1 same ad that never changes.
Is this normal?
btw I don’t see a refresh rate variable. could that be the problem?

Most probably, that ad is the only ad that corresponds to your region and ad type at the moment.
The refresh rate variable is in the admob site.

@AFTW, guess you need check “Enable test mode” in Services Ads of Unity editor.