This is an advanced and pretty specific need, but good to have in your toolkit.
On bbpress 2, if you set a forum from "Public" to "Hidden" or "Private", then back again to "Public" there is a bug that will hide the all the public forum posts from general users! Here's a great solution (posted by mjpg): Install the WordPress plugin "User Role Editor" and set the extra capabilities of the user role right there. SOLVED!
Thank you for sharing mjpg!