Post icons change colors

Let me see if I can explain this appropriately...

When you log on to my forum, the sub-forums that have new posts have their "new posts" icon (red circle with lines in it) showing. Let's say there are five sub-forums like that.... I go in to one of the sub-forums, read the new post, browse back, and now none of the forums are marked with the "new posts" icon (as though I've read every new topic); they all have the "no new posts" gray circle with lines in it. Why does this happen and how can I make it so that the "new topic" icon is always there until either the user ends their session, a reasonable time limit has passed (such as an hour) or all of the new posts have been read?