add the following code to the failure page to write the request being sent to and the response coming back from authorize.net:
<?php
@session_staat();
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>";
}
?>
Note, the full request and full response will contain sensitive info that should be removed before posting here.