Many web seminar invitations inevitably go to somebody’s equivalent of anonymous@hotmail.com, which means that often they end up in in-boxes that are checked infrequently. 

This means that as lead times have gotten shorter, there is a greater chance that infrequent anon checkers get your invitation too late – either too close to the event date to act upon it, or just plain too late.

Make sure the ‘register’ link in your invitation redirects to the web seminar archive.