The discount relies on session variables, so to troubleshoot, we need to know what those session variable values are.
on the cart page, add the following code just after the <body> tag so we can see what the sessions are being set to
<?php
echo("PromoCode: ".(isset($_SESSION['PromoCode'])?$_SESSION['PromoCode']:"")."<br />");
echo("PromoAmount: ".(isset($_SESSION['PromoAmount'])?$_SESSION['PromoAmount']:"")."<br />");
echo("1stDiscUsed: ".(isset($_SESSION['1stDiscUsed'])?$_SESSION['1stDiscUsed']:"")."<br />");
?>