All credit/debit cardholders are subject to validation checks and authorisation by the card issuer. If the issuer of your payment card for any reason refuses to authorise payment, then you will be notified of this immediately at the checkout.
A common reason for orders declining is due to the billing address details being entered incorrectly. Please ensure the 'Billing Address' section is the same address as printed on your respective bank statement. For full details of why the payment was declined, please contact your card issuer.