Vistors logon but unable to comment

Even if a user is logged in they get “You must be logged in to post a comment.” at the bottom of all posts.
“logged in” appears as a link, but the link just takes the visitor back to the page for the individual post. We’ve tried with a couple different usercodes. We’ve got the allow commnents option turned on, the administrater must approve turned on as well as the Comment author must fill out name and e-mail option.
Thanks