You should put the Authorize.net response code on the checkout failure page to determine what the problem is. Here is the code to add where you would like to see this info:
<?php
require_once("WA_eCart/Adv_CO_Scripts/AuthNet_PHP.php");
?>The full request is: <?php echo(AuthNet_Result("Full Request")) ?><br/><br/><br/>The full response is: <?php echo(AuthNet_Result("Full Response")) ?>
Please post back with the results you are getting if you can't resolve the issue based on this info, make sure to remove any sensitive details like credit card numbers.