Why are there no GAM Line Items associated with Empty VAST Response errors?

GAM Line Items are not associated with the Empty VAST Response error because this error sits in the Response zone.  Watching that receives details of the GAM Line Items once / if the sequence gets to the Playback zone and the ad has been successfully delivered.  

Watching That breaks down errors into 3 specific zones:  Request, Response, and Playback.  Each of these zones is assigned to a specific phase in a Video Ads Life: 

Request - This tends to be mainly caused by client-side code and components (player, IMA SDK, and custom scripts) 

Response - This tends to be mainly caused by the domain of the primary ad server and the participating SSPs and DSPs 

Playback - This tends to be mainly caused by the rendering of the ad itself in the video player 

For more on these Zones and our Video Ad Flow Map, please find further details here: https://blog.watchingthat.com/introducing-the-video-ad-flow-map