This is Elias from Shopify Support. Thank you for reaching out.
Sorry to hear that one of your customer wasn't able to checkout despite using different cards.
You can view the timeline of their abandoned order to find out why their card was declined. You can do this by clicking on the line that displays the declined payment (in red), the field will expand and provide details as to why the payment was declined. However, if other customers aren't having issues checking out, then we'd recommend having the customer contact their bank to resolve any issues they're having with their credit card.
Have you received any other reports from customers who aren't able to checkout? Or, does it explain why their card was declined in the timeline?