Typically with these payment errors it's either due to lack of funds or a mismatch between the shipping address keyed in, and the one tied to your crediting account.

However, if both are okay - the third common reason would be the fact that your crediting source is denying the transaction. Typically with international transactions, your credit card issuer might suspect it of fraud and in order to protect you, they would have to block the transaction. You would have to call them up to see if that's the case.

Once they've authorized the transaction, all you have to do is re-order and the purchase would now successfully go through :)