I posted a meta question a couple weeks back about receiving notifications sporadically or not at all from forum activity. I think I may have moved a step closer to the problem origin:
This morning I received an email saying there were 3 answers to a question I posted yesterday, even though the question and the 3 answers were posted 5 days ago. But the notification system is not the problem: when I visited the web site to view the question and the answers they all claim they were posted yesterday, which is flatly false!!
So sometimes questions and answers do not get fully "registered" in some sense; when they finally do, that changes their datestamp and that is when a notification goes out.
(For reference, my previous question is available here.)