A little more info is needed. Do you mean pre-paid cards / gift cards / credit cards ?
Some pre-paid cards and gift cards do not allow transactions for gambling.
That is most likely why then but it denied my bank card also. A bit of a head scratcher. I can deposit to bitcoin sites though.
Some will allow for online purchases but not gambling.
Also, sometimes you need to register your card. Go to the web address on the back of the card you are trying to use. There you will be able to register it. Some pre-paid cards just require a postal code to be registered others a little more information. One of the prepaid cards I used got declined before and after I registered my card I was able to deposit with it. You can also see your balance at that web page free of charge.
Vanillia pre-paid cards seem to always work for me ( without having to register them ) when I do use pre-paid as an option. Just make sure you read the information on the card first to make sure it doesn't say something similar to ... can't be used for gambling / online transactions.
They work on the WPN?