Please, check on the Stripe dashboard by entering the user's email:
Go to the Payments section.
If you see an FAILED, click on the message.
Read the error and inform the user
The reasons of Payment declined by customer's bank are:
The bank returned the decline code incorrect_number
The bank returned the decline code 'expired_card'
Stripe blocked this charge as too risky.
For more information: https://stripe.com/docs/declines#bank-declines