A payment can be unsuccessful for various reasons.
When paying with a creditcard it is possible that you are going over your limit when the payment will be accepted. It is also possible that the 3D secure is incorrect (those 3 digits on the back).
For countries where Klarna is accepted, your order can be cancelled because of too many outstanding orders or because Klarna detects a reason to cancel your order. Unfortunately we have no control or insight in the reasons why Klarna is refusing certain orders.