why doesnt my american express debit giftcard work?

Just purchase a new American Express giftcard. It is prepaid. I have only used Visa. I also have noticed that the card setup is different. Ive tried cahnge my paymeny method in the iTunes store to my american express but it wont let me. iTunes claims that my bank statements are incorrect. I do owe a balance which was my intial reason for purchasing the card. Is this issue have something to do with that owed balace or me not entering in the card info the right way?