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You don't allow access to your forums without people registering so I can't see your forums. But what you describe may be that your forums are set to thread view instead of flat view. You can make the default view for your forums be flat by going to system admin > preferences > general settings and choosing flat for your comments default view. This will affect not only comments but also forums.
Plus each user can edit their own accounts to choose flat or threaded default views for comments.