Customer having difficulty buying download

I had a customer contact me to let me know he is trying to purchase two downloads and has tried two separate credit cards and cannot complete the transaction. I don't know what type of message, if any, he is receiving, and have sent him an e-mail to find out. There is no order showing up in my Sales History. He tried Mastercard and Amex.

Any ideas what could be happening? I don't know how to direct him.

Thank you,
Lisa