If you have asked for notifications of a particular thread, you'll receive only 1 email alerting you of a new message posted on that THREAD while you are away from the forum. The moment you login or visit the forum/the thread, you'll again receive notifications when new posts are made on that thread.
Same is the case with FORUM section notifications. You'll get notified about a new thread created in that section only once and till you don't visit that section again, you won't receive the next notification.
Hope this helps.
Thanks a lot ! that explanation does seem to fit the 'randomness' of the emails I was receiving, I am permanently logged in to Bitcointalk and so I will just re-visit each thread in order to receive more email notifications., or alternatively log out and log in again.