A possible cause is the Ad response doesn't have a media file that can be played on the HTML5 player.
To troubleshoot: 1. Get the network logs. 2. Identify the Ad Tag. 3. Follow the ad tag workflow in case it's a Wrapper. 4. Look for the media files. 5. See if the media files are in the correct format. FLVs and SWFs cannot be played on the HTML5 player.