ok, add the code to the failure page:
<?php
if(!session_id()) session_start();
echo "<p>Auth Net: <br />";
echo "Full Request: ".(isset($_SESSION['WAAuthNet_postedValues'])?htmlentities($_SESSION['WAAuthNet_postedValues']):"n/a")."<br />";
echo "Full Response: ".(isset($_SESSION['WAAuthNet'])?htmlentities($_SESSION['WAAuthNet']):"n/a")."<br /></p>";
?>
after the body tag.
This will write the full request and full response from the gateway to the screen for troubleshooting.
NOTE: The full request and full response will contain sensitive information that should be removed before posting here.