Authorize.net problem with invalid credit card message

[filyawd]

Okay I have installed the FREE Authorize.net plugin and well this is a run down of what happens.
 1. I add a new invoice to a client
 2. The client logs into their account and clicks the view invoice link
 3. then when they open the PDF file to view the invoice and click the PAY NOW link
 4. They are redirected to " pay invoices page "
 5. then they are directed to the " secure payment form " and this is where there is a problem.

When the customer fill out all "correct" information on this form a message at the top of the page states " invalid credit card "
and have tried this on three different cards. " and the box for the card number is highlighted YELLOW "
Don't really understand why this is happening I really need some help and advice.
The form doesn't have a place to choose if it's a visa or mastercard or anything like that.
I live in the US and didn't know are UK cards different from US card IDK .... I'm really at a loss at the moment. I have tried test mode and live mode and both do the same thing.

[netshine]

This happens when authorize.net reports an error, however, it should also include an additional error message after 'invalid credit card: '. If there is no additional message, it is because no error message was reported by authorize.net. The first thing to do is to check and double-check all of the payment gateway settings (Login ID, Transaction Key, and MD5 Hash Value in particular). If you are certain the settings are correct, please submit a support ticket as I will need access to your site to insert a line of code to check exactly what data is being sent to and returned from authorize.net.