Many times an advertiser will send a VAST wrapper to a publisher. The publisher needs to unwrap the VAST to determine if it actually contains a video ad or not. The publisher assumes they have received a filled ad, but it is not until they have unwrapped it that they can know for sure. If they unwrap it and find an empty VAST wrapper then there is no filled ad to display. So if the publisher or advertiser counts the VAST wrapper as a filled ad and it turns out to be an empty VAST wrapper then it will lower their video use rate.
Another common error that affects the use rate is the media file type in the VAST. Many advertisers send a ‘flv’ or ‘swf’ file type which plays fine on desktop but is not able to be played on most mobile devices. The advertiser may send a filled video ad to the publisher but if the file type cannot be played by a mobile video player, than it will never turn into a video impression and will negatively impact the use rate.
Although there is a published VAST standard, some advertisers still send VAST that does not contain all of the required elements. Depending on which elements are missing a VAST parser or VAST video player may or may not be able to play the video. Some VAST parsers are more forgiving than others and will still play the video if the essential elements are present. If those necessary elements are missing though, the advertiser and publisher may count it as a filled ad but it will never turn into a video impression thus lowering the video use rate.
The last common cause for lowering the video use rate are timeouts. Timeouts can be caused by the ad server taking too long to send the VAST to the publisher or the publisher receiving valid filled VAST but the video file takes too long to load. The video file could timeout due to a slow connection or the video file could be very large and is slow to download. Desktop can accommodate much larger video files, but smaller optimized ones need to be used for mobile. Once again, the publisher may receive a filled video ad, but if there is a timeout then it will never turn into a video impression.
An advertiser may believe that they are sending a high percentage of filled mobile video ads to a publisher, but if they are not turning into video impressions than the advertiser may believe the problem is with the publisher. But the advertiser may be returning VAST that the publisher is not able to display for several reasons, so it is important for a publisher to have proper error detection and logging in place so they can track down the cause of the failure. Without adequate error logging in place the low use rate could turn into finger pointing between the publisher and advertiser, with each believing the other party is at fault.