Pre-paid mastercard on Steam?

So I bought a $25 mastercard today hoping to get a few games off of steam, I've used the same kind of cards on steam before but for some reason it's not accepting it this time. I've registered the card with my postal code and have put the same information I've put in for previous cards (replacing the old card numbers with the new one of course) and it keeps denying it. Anyone know why it's doing this?