From programming perspective, this probably is the most difficult to get wrong. It is only an internal messaging system.
If this is genuine intermittent error, I only can think of database server problem. Maybe it take ages to insert one record or the server just abandon the request.
Low, the non-delivery problems of the eBay messaging system have been around for
years. My seller friend scored a neg because a buyer's messages to them did not get through. The item was damaged in the post, but the feedback was the first they knew of any problem.
This is one issue that is difficult to prove - since you need an alternate communication path to find out what messages did not get through .... and eBay have made very firm moves to prevent that.
Tello's comment is one that I could not even begin to argue against ... even if I was motivated to do so.