It's hard to say what could go wrong, but most likely, you will continue to have no success unless you contact iTunes Store support as instructed.
However, if the address that you are entering for the credit card doesn't match the exact address on your credit card statement, that might cause a proble,. Sometimes if you use an abbreviation and the card statement doesn't list the address with an abbreviation that might cause an issue. Are you sure that you entered the 3 digit security code for the card correctly? That can cause problems as well.