Have you contacted your Credit Card companies Customer service to ensure there isn't some online purchase protection they have on your card that might be interfering with the purchase. SE and the country the purchase is being billed from might be seen as suspicious by your CC companies systems and blocking the transaction.

not saying this is the only reason but could be. Also is your CC a true Credit card or a debit style Mastercard/visa card. make sure you have enough $$ for the transaction.