Hi there,
Are you experiencing these difficulties on iOS or Android?
If possible, please refrain from requesting any video ads for the next 24 hours. After that time has elapsed, go ahead and request an a video ad, and when it loads, please try interacting with the ad as much as possible.
Sometimes, frequent video ad watchers, who do not interact with ads (i.e. checking out their website, or at least getting to some type of offer wall) will be registered as someone an ad network doesn't want to serve ads to. In that case, sometimes we run out of specific inventory to serve to that particular watcher.
Hope that clarifies things!