Are the going to the pp_checkout_failure page?
if so add the following code on the failure page to get the response back from paypal:
<?php
if(!session_id()) session_start();
echo("Auth request: ".isset($_SESSION['ECO_Auth_Request'])?$_SESSION['ECO_Auth_Request']:""."<br /><br />");
echo("Auth Result: ".isset($_SESSION['"ECO_Auth_Result'])?$_SESSION['ECO_Auth_Result']:""."<br /><br />");
echo("Get Request: ".isset($_SESSION['ECO_Get_Request'])?$_SESSION['ECO_Get_Request']:""."<br /><br />");
echo("Get Result: ".isset($_SESSION['ECO_Get_Result'])?$_SESSION['"ECO_Get_Result']:""."<br /><br />");
echo("Preocess request: ".isset($_SESSION['"ECO_Process_Request'])?$_SESSION['ECO_Process_Request']:""."<br /><br />");
echo("Process Result: ".isset($_SESSION['ECO_Process_Result'])?$_SESSION['ECO_Process_Result']:""."<br /><br />");
?>