Hmm... I'll have to look into that.
I've always just gone by the rune icon colors to the left of each board section.
It's colorless when all posts in a section are read, but it will have color when there's something new. (I had to create separate images for that to work)
Argh.. I just checked.. you mean the "Show unread posts since last visit" link? ..it doesn't appear to work.
I thought maybe if a user was always logged in it wouldn't "take effect", and you had to log out and back in, but I did that and it still made no difference.
Must be a glitch with the forum software, or, caused by my choice of theme maybe. I'll look into it.
But for now, just keep your eye on the rune icons -they'll let you know if there's a new post.