Clients can't make payment to my site

Has anyone had trouble with the payment from clients being declined?
I tried with two cards, pretending that I am the client.
The first time I succeeded making payment to my site. However, the same card was declined at the second time.

I called paypal, they says it's your card's problem. I called the bank, they said they didn't even see any payment request making by paypal or by my site. My guess is something wrong with the site. Does anyone have any experiences/ advice/ troubleshooting for this issue?