Sounds like there is an issue with the PayPal Standard IPN hitting your server and telling it to upgrade the member.
It can sometimes take a minute or two for PayPal to do this. Sometimes an issue with your PayPal settings, your server or a bug in our code can keep PMPro from upgrading someone's membership level when the IPN event goes out.
First thing to check is that the email address you have set in the PMPro settings is the same as the one in your PayPal settings.
You can get more help with this at http://www.paidmembershipspro.com/