To debug this issue, please place this code in your failure page. This will give more information about the error you are getting and why it is sending you to the failure page.
<?php
if(!session_id()) session_start();
echo "Direct Pay: <br />";
echo "Full Request: ".isset($_SESSION["PP_DirectPayment_postedValues"])?htmlentities($_SESSION["PP_DirectPayment_postedValues"]):"n/a"."<br />";
echo "Full Response: ".isset($_SESSION["PP_DirectPayment"])?htmlentities($_SESSION["PP_DirectPayment"]):"n/a"."<br />";
echo "Express Authorize <br />";
echo "Full Request: ".isset($_SESSION["ECO_Auth_Request"])?htmlentities($_SESSION["ECO_Auth_Request"]):"n/a"."<br />";
echo "Full Response: ".isset($_SESSION["ECO_Auth_Result"])?htmlentities($_SESSION["ECO_Auth_Result"]):"n/a"."<br />";
echo "Express Process: <br />";
echo "Full Request: ".isset($_SESSION["ECO_Process_Request"])?htmlentities($_SESSION["ECO_Process_Request"]):"n/a"."<br />";
echo "Full Response: ".isset($_SESSION["ECO_Process_Result"])?htmlentities($_SESSION["ECO_Process_Result"]):"n/a"."<br />";
?>
If you still having issues, please post back with the respond with the output you are getting on the failure page. Make sure to remove your credential information before posting.