When someone makes a purchase on your site, once they click on the "Place Order" button, the credit card / billing information is then passed on to your Payment Processor company, who then verifies and attempts to authorize the transaction.  If the transaction is declined in the process, your payment processor company then bounces back an error message to your site.  However, this message is generated from their end and as a result, they should be able to provide you with the exact reason as to why the transaction was declined (ie: invalid credit card, etc...).  We do not have the ability to provide you with this information since we are not the ones who verify the transaction.  Please check with your payment gateway company or your credit card company regarding the reason for the declined transaction.  Once they provide you with a reason for the decline error, you can then provide us with that information so we can advise further.  Thanks!  We do apologize for this.