I have had this happen on my site.
I ran into it when the site cookie did not work.
When I log in to the site @ http://www.yoursite.com
and then enter the website with http://yoursite.com
. The site made me re-log back in. If I added the "www" in front of the new link I was loged in as I should be.
I fixed this by:
I edited .htaccess on the webserver to force members to http://www.yoursite.com
no mater how they enter the website.
(make sure Rewrite is on with your web host.)
Content visible to verified customers only.
Since the edit I have never had any problems.