Membership Upgade Problem

Hi all and thanks for all the help. I was wondering if anyone knows how to fix the membership upgrade. When in membership paid mode the system does not upgrade members to their respictive membership level. Site is configured with Paypal and there is no problem with the payment process. It works fine and Paypal redirects members to site. But then the membership upgrade status does not change. Your hepl is greatly appreciated. Regards, Tony.