I have responded to @vandervelde
regarding this issue.
Unfortunately, if a card is not being authorised, then there is no other option but to use an alternative method of subscribing (PayPal or check/cheque).
2Checkout, our payment processors, can also be contacted directly and explain exactly why a card is not being accepted. Their helpdesk numbers are as follows :
US & Canada: 1.877.294.0273
United Kingdom: +44 (0)808 234 0487
But do keep in mind these guys are trying to prevent fraud, and even thought typically this isn't the type of site that anyone would use a stolen card to use, they have to treat all cases equally - so they can seem over zealous.