There are a number of different reasons for the bank or payment processor to block the payment. Some of the most common are:

– The credit card number or expiration date is invalid.
– There are insufficient funds in the cardholder’s account.
– The cardholder/location or activity is suspicious.
– The bank’s fraud rules which can take many other factors into consideration.
Many times it is accompanied by other clues, but the only accurate way to know is to contact the issuing bank for clarification.