for troubleshooting, create a new php page, add a cart display to it, on the second step, set it to show all columns
then add the following code at line 1:
<?php @session_start(); ?>
to force php to look for the session, and add the following code in the body to write the session contents to the page:
<?php var_dump($_SESSION); ?>
send a link to the product page that should trigger the discount and a link to the new cart display page so i can see what is going on.