I don't know if you can do anything about it, but when I first come to this forum, it will show a special icon "Foler New" for sub-boards that have new topics. If I open up such a sub-board, it correctly highlights all unread topics as New.
If there is more than I New topic, and I read only one of them, then when I return to the home page, that sub-board will not be flagged as holding new topics, even though it still has new/uread topics. If I remember which sub-board it is, or stumble upon it, then the un-read item will still be marked New in the sub-board.
So it kind of works, and is kind of broken. I think they call that a bug.
Or a memory game! Which sub-board has the new posts?