Please refer below for the cause and what you can do to solve the issue.

■ Possible Cause

 1. Exceeding the limit on the number of ad views

 2. Real-time network instability

 3. Ad-blocking apps installed

 - Reboot your device

 - Wait a while if you exceeded the number of ad views

 - Delete applications from unknown sources

 - If you are using Google Play [Settings > Google > Advertising > Reset Advertising ID]