You can look up error codes here: https://developer.paypal.com/docs/classic/api/errors/direct-payments/
This error is: 15005 Processor Decline
This transaction cannot be processed. The transaction was declined by the issuing bank, not PayPal. The merchant should attempt another card.
I'm not sure why it would work with paypal terminal and not on the site. You could ask paypal about that one. Are you sure they used the same card? The only one that could really answer the question about why it was declined would be the user's bank. It was the bank that declined it.