I have the same problem. I have never received a single email notification regarding subscribed threads. I have only ever asked one question here on AB, but have posted over one hundred in an attempt to help others, mostly in the technology section (and occasionally quizzes & puzzles). I think people think i am ignoring them when I don't respond promptly, but it is a nightmare keeping up with the various threads manually.
I am actually in dialogue (by email) with tech support regarding this issue. In fact I have been for several weeks, still without resolution. Initially, they suggested I was not subscribing to the threads, then that I was unsubscribing, the latest is that my email provider (googlemail) is blocking them. Whilst this may be possible, I get the emails from tech support fine, which I assume is from the same domain (theanswerbank).
So, I have changed my email address to another I have and we'll see, but I have already subscribed to various threads this morning, to which there have been subsequent replies and I have had no email, so it is looking grim.....:(