Just look at your self serve as a vending machine. First you add the money then you make your selection, confirm and submit your payment. Then your selection drops into the bottom right corner of your overview page.
It's either you haven't completed the add on page process, or your phone uses data for OS updates, software updates or other apps, and you run out of data.
Are you buying a data add on? If so, it is a 2 step process. You add the funds to your account and now you need to go back into the add on page and make the purchase.
