Read the error messages and responses from the gateway when the payment fails. To do this:
Navigate to the Customer Overview page and click Payments.
On the Payments page, click Details under the Actions column.
The response from the gateway for the transaction failure is the value of the authorization response field.
Details of failed payments are also available in the gateway reports. To download the gateway report:
Click Reports in the side menu and under the Other section, click Gateways.
Provide a date range and gateway, and click Generate. You can also click the Export icon to download the report.
Select basic customer details and click export
Review 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.