Random customers calling, can't place order with us online

Hello,

We are having an issue.

We have had 4 calls within 24 hours from customers stating all the same things:

They attempt to place their order online.

They get to the checkout page, and cannot complete the transaction.

They receive a message to contact us, the payment cannot be completed.

They call the bank, and are told there is nothing wrong with their card.

So then they call us and we can (sometimes) process the order. Twice, we could not help the customer, and once, we were able to process (using the same card). The 4th customer opted for a PayPal payment request. It would be interesting if he can make that payment using the very same card that was declined, but of course, I won't have access to that particular information, as we don't see customers' methods of payment when they use PayPal.