I’ve noticed that too.
It seems that you have to specifically mention the person you are replying to in order for the post to be seen as a reply rather than just another post.
Edit: I replied to @mc3man’s post using the the ‘Reply’ button at the foot of his post and my reply was seen as just another post in the thread that he started.
Edit 2: I was wrong. Even that doesn’t seem to work (now).