prepaid debit cards not accepted?

Is it a Vanilla Visa? If so, that's probably why it's being declined. You no longer have to register those cards, and most companies require a zip code/street address on file for the card in order for it to go through. Green Dot Visas are prepaid and are a much better option for future use as you have to register those online.

also, keep in mind that sometimes you'll need a small dollar amount on your card in the event a temporary hold is placed to make sure the card has the necessary funds for the payment to be processed. So if your bill is $120, you'll need $121 on said card.