How to find details of why customers purchase was declined?

Customer’s payment can be declined for various reasons. Most often it is an issue with customer’s card or customer’s bank , as they have automated systems that determine whether to charge or not to charge. Even when customer says the card has been successful previously, a future charge can be still declined.

So from your end, you can only suggest if asked to use a different card, a different payment mode or ask customer to contact the bank.