There can be many reasons why a card is declined. To determine the reason for a decline, you will need to review the transaction response from your payment gateway account, Authorize.net. Authorize.net will create transaction reports for all your transactions - both your successful and failed transactions.
See the Why was my transaction declined? article from Authorize.net for details on how to review the transaction response from your payment gateway account. Compare what you see on your transaction to the common decline messages listed in this Authorize.net help article.
Still confused on why the card was declined? Get in touch with us or Authorize.net support.
Also - If you ever see a decline message that states "General Error", let us know. That can sometimes indicate an issue with your merchant account.