That can be caused due to a few reasons.
The main reasons are mostly 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 hit the "Submit a request" button in the upper right corner and let us know about it.