- The bank's fraud rules (which consider various factors that are not made public) have been triggered.
- The bank may have placed a temporary hold on the customer's card.
- The purchase session may have been locked due to multiple declined payments.
- The seller is located in a country different from that of the card issuing bank.
- Ask the customer to contact their bank, explaining that they are trying to process a payment. The customer can ask the bank to allow the payment.
- Ask the customer to try again at a later time. The issuing bank may have only placed a temporary hold on the card.
- Ask the customer to use an alternative credit card.