In order to determine what may be wrong with your Authorize.NET checkout you will need to add in the full request and full response to your checkout failure page. Here is the code to add to this page:
<br />The Authorize.net full requst is: <?php echo(AuthNet_Result("Full Request")) ?><br />
The Authorize.net full response is: <?php echo(AuthNet_Result("Full Response")) ?><br />
Put your cursor on the page where you would like to see this information, then go into split view and paste the code in. When you test the Authorize.NET checkout you should see the full request and full response here. If you have trouble understanding the error please post back with this.