Integrate w/ other user database app/tables

I am having the same problems but just this minute discovered the reason so thought I would post in case its the same for you.

On my site, I switch between the forum pages and my main page.  I kept losing the session variables between the two which meant that the member API never worked.  The reason is that on my main page (called footyweb.net/tipping) I was referring to the forum as "www.footyweb.net/forum".   ie. with "www" prefixing the site name.

This might be something you could check with your site.