i have also try this to see what could be the problem
For Direct Payment add these into the checkout_failure.php
<?php echo(PP_DirectPayment_Result("Full Response")) ?>
For Express Checkout add these into the pp_checkout_failure.php
<?php echo $WA_PP_ECO_DoResult->fullResponse; ?>
When you test the checkout after applying this you will get the full request that was sent and the response. In the response you should see all the information about the error message.
and since i added thos lines im getting:
Fatal error: Call to undefined function PP_DirectPayment_Result() in /home/content/xx/xxxxxxx/html/checkout_failure.php on line 107