It may not be anything to do with your newsreader. It's by no means unknown for people to start a new thread with the same title as an old one, rather than just replying to a message. No newsreader will regard that - and any replies to it - as part of the original thread.
I've even had to do it myself on occasions, when I've tried to reply but have received an error message to the effect that the thread is too long - so starting a new one, and cutting and pasting text from the old one into it, has been the only way to go.