This could be due to a couple of possibilities:
- The incoming emails could be tagged as junk or spam by your email provider, or internet security software. Check your SPAM and/or Junk folders
- When you subscribe to a forum, you will get emails relating to new topics posted in that forum, but you will not get any replies to those topics unless you have subscribed to that individual topic.
It is organised like this:
forum -> topic -> replies
