Sounds like you have to make them "Allow Mods and Admin" to start new threads, polls and reply to topics in each of the private forum's settings. From the Tutorial:
Edit Settings - This allows you to edit the main options and preferences for the forum. After selecting this option, you will get a screen with various text boxes, select menus and checkboxes. Here's the list:
Forum Name - This is the title of the forum, displayed at the top of pages, and in the quick-jump.
Description - This is the text displayed underneath the forum name on the main board page. You can use text or HTML here safely.
Category - This is the category displayed in the category bars on the main board page.
Make this new forum a subforum of an existing forum on the left? - If you select a forum from this list, then the current forum will become a subforum of the selected one, meaning that it will show as a forum inside the forum you select, and not show on the main page.
Hide Forum? - Checking this box will make the forum invisible (it won't be shown on the main page) to anyone except admins, and people you specify to be able to see the forum. See Edit Users on how to specify that certain users can see hidden forums.
Mod Display String - This is the text that displays the moderators names, if you have selected to have that displayed. You can add names, remove names, or format them with HTML. Changes that you make here do not affect the powers that any users have - it's just how it's displayed. Who can start new threads? - Choose one of the options for who you want to be able to start new threads in this forum.
Who can start polls? - Choose one of the options for who you want to be able to start new polls in this forum.
Who can reply to existing threads? - Choose one of the options for who you want to be able to reply to existing threads/polls in this forum.
Who can view this forum & the threads in it? - Choose from one of the options for who you want to be able to access (read) this forum. Choosing anything with "Specified users" makes the forum a private forum. See
for how to allow people to access private forums.
This forum is an advert forum? - Checking this box will make this forum an advert forum, meaning that users will have to have a certain number of posts on your board before they are able to post in that forum. If enabling this option, enter the number of posts needed in the text box below.
Posticons - Check this box to allow posticons in this forum. Posticons are the images in a column to the left in the threadlist, and at the top of each post, which give a basic idea of the content of the post, by the image's expression.
Personal Icons - Check this box to allow personal icons in this forum. Personal icons are the small icons that appear to the right of usernames in posts.
Personal Pics - Check this box to allow personal pics (avatars) in this forum. These are images which the user can choose to display under their username with posts.
Sigs - Check this box to allow the use of signatures in this forum.
That is all the settings you can adjust in a given Forum-the italic ones are the ones I think might be needing adjusting.
Let me know if that works.