When you mark a forum as "Read" it only marks those posts before you marked "Read". New posts will show as "unread".
Do you mean you wish to "subscribe" to that forum again?
You can click on "Forum Tools" (in the blue bar above the threads)
Then click on "Subscribe to This Forum" (in the dropdown menu).
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They will now show in your User CP under Subscribed Threads when any new posts are made in that forum.