Try enabling the option “Show Ads when brave is not in use” on Android.
you can find this option in Setting/brave rewards/Show Ads when brave is not in use.
It will show you ads even if you background Brave. This is for Android only.
Ads are serve when we gain users attention. This is as much information regarding this request that we are willing to share regarding this as information like this about the inner workings of our system often help fraudsters game the system.