Make sure to use the identical address when making a card payment. Some merchants use AVS (Adress verification via credit card networks, they check if the billing address of the card matches the address / ZIP you gave them.)
Also, have you checked the security settings for the card? Online payments can be disabled there.
Is your account fully verified? Does the merchant accept prepaid cards?
(I assume it is not a general problem since you wrote that the 2nd payment with the 1st merchant went through.)
And this: