I've this for ages it'll happen for a day or a couple of hours, and then other times it'll be fine. It's annoying, but a good workaround I've found (if you browse the forums the way I do) is to hit New Posts, read all the threads you want to, then go to Quick Links and select Mark Forums Read.
Above that is Today's Posts, and I usually hit that afterwards and read any new threads that have slipped past since I last pressed it, and then New Posts works for just the posts after that.
I find I have to browse like this for a day or so at times. It just seems to work intermittently.