There are many different reasons why a payment does not succeed, however many payments fail for good reason and do so to minimize the possibility of a fraudulent payment. The two main categories of failed payments are Declines and Blocked payments.
There are three possible reasons why a credit card payment might fail:
Typically, with a declined payment there will be a decline code sent by the issuer bank. To view a list of decline codes, and what each code means, view the Decline Codes page.
You can read more about card declines and how to address them on the Declines page.
Certain failed payments might be preventative measures working in your favor to minimize the possibility of a fraudulent payment, that otherwise could result in a dispute.
Examples of this are cards that are picked up by Stripe Radar as fraudulent, or that match custom Radar rules created by you to filter out transactions you do not want to accept. Payments that are picked up by Radar rules will be blocked as part of fraud prevention.
You can read more about built-in Radar rules on the Radar Rules page.