Why did a customer's payment fail?

Check the payment details. It will contain the message received by the gateway which will contain a hint as to why the payment failed. 

Common reasons are as follows:

• There was an insufficient available balance on the credit card. 
• The card has been deactivated.
• The card has been locked for fraud. 
• The gateway service provider has detected duplicate transactions. 
• The gateway could be misconfigured. 
• AVS verification has failed

In most cases, your end customer will need to discuss this with their credit card issuer, or take additional steps to provide a working payment method in order to pay for the services you are providing them. 

Further reading:

On a failed payment or validation, what does this "Reason for Failure: ..." error message mean?

Collecting a Manual Payment

How do I view a customer's Payment Method information?

How do I record a payment that took place outside the Fusebill platform?