From the Model Mayhem FAQ:

Why was my credit card declined?
You should receive a notice that states why your card was declined, but the most common reasons your card issuer will decline a credit card are as follows:

  • Do Not Honor. This is your bank refusing to process the charge. The reasons may vary from bank to bank, but this is usually issued because your bank is trying to protect you. You must contact your bank in order to resolve this issue.
  • Credit Floor. This error means that there was no credit available for the charge to go through. You will need to contact your bank to resolve the error.
  • Invalid Credit Card Number. This issue either relates to an incorrect credit card number, invalid billing address information, or, the credit card type selected was wrong. Double check that all the information provided is correct and try again.