Why was my customer's transaction declined?

A customer’s bank normally has fraud prevention systems in place that can decline a transaction if the provided card information is incorrect or invalid. The issuing bank examines data, such as your customer’s account balance, spending habits, card expiration date, card verification value, etc. If the card information seems correct, you can recommend your customer contact their bank for more details. In that case, you might notice a "do not honour" payment error code on your transaction list. 

Alternatively, you can review the other decline reason codes here.