There are a few reasons why your card might be denied:
The Credit card company processing your card, for whatever reason has deemed that there are not enough funds or too many transactions on your account to process this order. You will need to contact your credit card or bank company.
The credit card information you entered did not pass our Billing Address Verification Check. When we check a card for available funds, we also check to make sure that the billing address of the card matches what the user entered. This is one way that we protect our customers from fraud. If the billing address doesn't match, then the order gets declined.