Right now some of the new posts that are made that show a red icon on the forums main page is not showing the red icon indicating that a new post has been made in that forum that you haven't read yet, its a server side issue and is affecting most all the forums on this sever. They are aware of the issue and are working on a fix. Its out of my hands the server technicians are working on it.
If you want to be sure you don't miss a post by someone you can go to there profile page and click "statistics" and then click "View all messages posted by" this user.
I'll update this post when its fixed.