Ah, I didn't know that Kobo used WorldPay.
I've done a little digging and found other people complaining about WorldPay rejecting credit cards.
http://support.moonfruit.com/entries...ment-processor
On this website people score WorldPay 1 out of 5, you can read many complaints about them.
And
this site explains some of the sorts of errors and reasons why WorldPay might reject a credit card.
I'm a web designer and have worked on several websites that accepted credit cards. Most websites don't deal directly with credit card companies, instead they go through a credit card processing company, such as WorldPay. The processing company run various automated fraud checks before passing on your CC details to the actual credit card company (i.e. Visa). The end website (i.e. Kobo) get back a simple "Ok" or "Not Ok" message to say if the transaction went through ok or not. Very often if the transaction didn't go through then they don't get a reason why, or if they do they just get a status code that might mean something to the CC company or the processor company, but is usually meaningless to everyone else.
So I'm not that surprised if Kobo don't know why a particular transaction was rejected. And it's almost certainly not Kobo's fault for the rejection. If you're lucky enough to get a transaction number from Kobo then you might be able to chase it up with WorldPay or your credit card company. In the past when I've had problems my credit card company was able to tell me that they could see the request coming in and how they responded, which told me if the fault lay with the CC company or the processor company.