If you upgrade, and enter a different email address than your already registered account, it creates a new account with your upgraded membership.  Then you try and login to your old account and it is not upgraded.  If you have done this, you will need to contact support. 

Alternatively, if you are reading this BEFORE you upgrade, please be sure to enter the email address you used when you registered your account, and your upgrade will be instant.  There are some offers on the site where manual upgrades are required, but you will be shown this at the time of purchase.  In these cases, your upgrade will be processed within 24 hours, usually much faster.