Make sure that you register the correct billing information with the prepaid or gift card provider. You can do this by calling the customer service number shown on the card. Make sure the billing information you enter in Roblox matches the registration information exactly.
If the transactions are not going through, please do not try to re-purchase. This can result in pending charges that can place a hold on the balance of the card for up to 10 days. The amount held will return to your card automatically and Roblox will not receive payment. If you have problems even after registering, please call the customer service number on your card to see why they are declining the charges.