add the following code on the checkout_failure.php page to show the full response and the full request:
<?php
if(!session_id()) session_start();
echo("PayPal Full Request :<br />");
echo("<code>".isset($_SESSION["PP_DirectPayment_postedValues"])?$_SESSION["PP_DirectPayment_postedValues"]:""."</code><br /><br />");
echo("Paypal Full Response :<br />");
echo("<code>".isset($_SESSION["PP_DirectPayment"])?$_SESSION["PP_DirectPayment"]:""."</code><br /><br />");
?>