You need to look at the payment provider log in the logs section of the control panel. That will tell you if there are any errors in particular.
Assuming you're using PayPal, if it's empty, then you may not have setup your PayPal account to enable IPNs. That would need to be done (which may involved changing your PP account type) and it should allow changes in the future. For any existing purchases, you'll need to do the manual upgrade options.