1. Verify that the form is in Active mode. If a form is in Preview mode, only the test credit card can be used to process transactions. Legitimate credit cards will fail.
  2. If the organization is using a custom payment processor (such as PayPal), it is best to check with that vendor first. Many times this is indicative of an issue with the payment processor.
  3. If the issue persists, Chat With Support and reference this article.  Please include example Payment IDs of failed transactions.