Is there any reason why notifications would not be sent?
I have tested by replying to topics between 2 users, however no notification alerts are sent.
Only shows topic update alert/highlight in front page list.
Also note that there is a very big conceptual difference between a reply to a topic (pressing reply button at the bottom or right side of the topic) and a reply to a specific post (quoting a post, pressing reply button on a post) or a specific person (a @username mention).