Problem Ads Video in the last 7-10 days

Since about January 27th we have been reporting from many users(like 10 at day min) in our games that they are unable to see ADS videos.
The app has had no problems in the previous months.
The message is that there are no videos available to watch. It's definitely not a software problem as we haven't changed anything lately.
Sometimes they manage to see 1-2 videos of the 10 daily but always trying a lot.
The latest version of Video Ads used is 3.5.0
Our fill rate varies from 90% to 97%.
What could it be?
Have others had this problem?
Thank you