[Zope] Problem with checking roles an authenticated user has

Hi, I have a site which includes a members area. When a member logs in, extra features are made available on all pages compared to a non-member. The problem is that sometimes the authenticated_user test I use does not come back true once someone has logged in. This problem does not occur all the time or with any particular page. It is being very unpredictable! This is the dtml-if (placed in the standard_html_header) I use to check... <dtml-if "AUTHENTICATED_USER.has_role('a_member') or AUTHENTICATED_USER.has_role('Manager')"> Any ideas what my problem could be? Thanks for any ideas Richard