Morning Roger,
If someone replies to an old post it automatically jumps to the top, in turn this notifies other members that another reply has been made, (unless they have ticked the box to follow that particular topic, in which case they would receive an email), however, I also have the ability to bump an old post back to the top, which I occasionally will do if the forum goes a little quite or if I feel that there is still more to discuss in a particular thread/discussion. Members can also pm me a request to bump a topic which I will also consider. Just thought I would explain how the bump process works.