There are a number of reasons why a charge can fail.
The customer's card issuing bank declined the request to charge their card. The customer may wish to contact their bank to request the charge be permitted.
The customer doesn't have enough funds available on their card. They may wish to add funds before trying to charge the card again.
The charge was flagged as possibly fraudulent and subsequently declined.
The card issuing bank requested additional authentication from the cardholder, which was not successful. This is possible only on charges that are 3D Secure enabled.
The card provided has expired. If the card is stored for repeat or recurring charges it's recommended to ask the customer to provide updated card details.
An error occurred while processing the card. This may be due to network issues at upstream financial partners, or other unforseen issues. Attemping to process the charge at a later time is recommended.