Ciar, the forum behaves a bit oddly like that if you don't log in for a while. Not sure the specifics, but I've noticed that if you don't log in for ages nothing shows as new until you have. Perhaps there's some kind of "unread posts that are older than X are displayed as new" or "unread posts that are older than X are displayed as new unless user hasn't logged in in X days" filters.
Anyone else having probs? Probably not much I can do if you are, but might be worth knowing what the common factor is