I've noticed almost right away that I just don't get e-mail notifications from QJ like I should, even with threads that I'm actually subscribed to. Over at toonzone.net forums, where I've been a member for over 5 years, they use the same system (vBulletin) and the notifications there are lousy for me, too. Extremely sporadic.
It's quite frustrating to go back to a thread you know you've posted in, and subscribed to, and see a bunch of replies that you had no idea were there. I'm sure I've forgotten about things because of this, too. This happens to me at any forum that uses vBulletin.