to determine why the Transaction is not going through successfully, we will need to know the response from the authorize.net server.
Add the following code to the checkout failure page after the <body> tag to display the response:
<?php
if(!session_id()) session_start();
echo("Full Response: ".$_SESSION["WAAuthNet"]);
?>
post back the response after testing a transaction, this will give us an idea of where to look for the cause of the problem.