I think the problem is something like this...
The appearance of the new post markers is controlled by the timestamp of each post, and the "last visited" date, which as others have noticed, isn't getting set correctly. However, when you log out, and log in again, the last visited date IS set. So the problem seems to be when you leave the forum without logging out - i.e. when the server times you out. I'm thinking that this is probably a bug in the IPB software, and while we wait for a fix, the work-around might be to log out when you have finished a session on the forum, and therefore having to log in again the next time you visit. A bit of a pain, I know, but it might just make life easier!