after payment with sandbox single paypal, the role does not change

We have two plans, one free and the other single paypal.
How the system knows which one to use? (price 0 is the free, for paypal is the first active gateway?)

The problem is that when we click the button for 'Single Payments', the PayPal process works fine, and takes me all the way to the PayPal 'transaction complete' page. But, the user level is NOT changed for the user.
Is something not working properly?