I had this problem in October and still having this problem today. I don't know why I can't use my US based credit cards to purchase flights, even if it's on the US website.
It keeps rejecting my credit card saying there's something wrong with my payment or it couldn't be processed, even though I tried 8 different kinds of amex, mc, and vs cards, debit and credit. Nothing works!
I ended up using a OTA, but hoping I don't need to make changes because it's such a hassle calling them up.