Here is a list of questions people have asked, and answers given. These are taken from the Need help? Got Advice? forum.
Help with the Forum
Forum topics not being marked as read
Q: When browsing into a topic, then going back to the forum via navigation links (not the BACK button), the topic is still being marked "Unread".
A: Try deleting your cookies and see if it helps. (Answer provided by RuthieG in this thread.)