the problem is domain switching
when you start shopping using the www address, the cart contents are stored in a session for that domain. When paypal directs you back to the site, it is not using the www address, it is leaving the www off, this is not the same domain as far as the session cookie is concerned so the cart is empty.
That is why my initial reply was to ask about the address in the address bar, I wasn't concerned with the page names, but the full address.
in the wa_globals.php page, make sure that the address you enter for the $WAGLOBAL_Root_URL is the full www address.