Sadly I can't pinpoint the issue or give correct "how to reproduce" instructions, but I have been missing a lot of emails lately from bitcointalk.
Notification emails on thread I'm subscribed to were always reliable for me ... but sometime in the last month or so, I have been finding out that threads I'm subscribed to have been updated, without any emails being sent to me. I thought maybe I accidentally missed a mail or two, but this pattern is far to consistent now to associate it with a mistake on my part (I practice
Inbox Zero, and I don't miss emails ... plus, my host is Gmail, and bitcointalk posts have never been classified as spam).
So, I have to conclude there is some bug in the notification email system, and would suggest that an admin/dev investigate this. Or we can just wait to migrate to the new forum software, whenever that happens.
For me personally, the bug is quite annoying ... I don't know if others have felt the same issues or not.