HostGator cannot know why your payments fail, but it could be due to lack of funds.
Also, many credit card companies are now requiring the customer security code on the back of the card to be submitted with every transaction attempt. Unfortunately, it is illegal for us to store this number. As a result, some cards will work the first time, but fail every subsequent time. The only options we can suggest are to use a different credit card, or login every month at http://gbclient.hostgator.com/ and pay your invoices manually (one-time payments).