I have found that sometimes you need to completelty leave the site and restart your browser for it to update the new posts count.
Example: If I leave my pc up on the site for two hours while it says there are 3 new posts, and then come back later and click on the Forum link, it may still say three even though there are more or less actually there. So I restart my browser and the correct number then shows up.
If this is happening when you go to the site from a fresh browser window, then i do know. Otherwise it could be something like I listed above. Possibly something with session id'* and cookies that isn't getting refreshed until the site is left.
Either way, that'* my two cents...