A few reasons can cause this issue.
The main reasons are poor Internet connection or simply that there are no reward ads to display.
If you encounter this issue, we advise switching between your device's networks (Wi-Fi/LTE/4G/3G) and check if still no ads are available.
If changing networks does not resolve the matter, please let us know by submitting a support ticket with as much information on the issue, and we will gladly assist.