There are several factors that might cause it:
• bad Internet connection;
• your local ad provider;
• exceeded the limit for watching ads.
That's why we recommend you:
1) wait a while for the ad limit to reset;
2) check if your Internet connection is stable and connect to another network if necessary.
I hope these tips are useful to you!
Feel free to contact our support via in-game settings in case any other questions come up.