Emails stop sending after a few comments

So this issue has been with us since the beginning. That is, i don't think it suddenly started occurring.

So the issue is as follows. after a few responses on a created ticket, emails stop going out to the ticket creator.
Typically the creator of a ticket gets to reply once or twice.
We allow any domain to create tickets, though typically they come from users in our own domain, and sometimes they already have accounts on our website, though this does not seem to have any correlation to affected users.

e.g.
Any User (U) creates a Ticket (always through email)
Assigned Support User (Su) Comments (on the Ticket's node) or replies to the Email
U replies to the reply Email
Su replies to Email or comments on the Ticket's node
U replies
Su replies to Email or comments on the Ticket's node
U doesn't receive the response.