Read the error messages and responses from the gateway when the payment fails. To do this:
Go to customer dashboard->payments
Click details on the failed payment
The response from the gateway for the transaction failure is the value of the authorization response field
Details of failed payments is also available in the gateway reports. To download the gateway report:
Go to reports->gateway
Enter the date range and gateway->click generate->click export
Select basic customer details and click export
Click download and the report will have the error messages for each failed payment that was attempted.
If the error message is not clear enough for the user to be able to resolve it themselves, contact your Gateway Services provider for more information/resolution. Alternatively, another valid payment method can be used to process the payment.