We check all your card details when you pay, so please double-check you've entered them all correctly. One mis-typed number and the transaction will fail - and it's very easy to mistype things!
If you're sure you've entered all the numbers correctly, please save your order and get in touch with our Customer Service team. We'll do what we can to sort it out as quickly as possible.
Sometimes - it's rare but it does happen - we get "communications errors" popping up between our computer and the bank's computer, and that can cause problems like this.