I have a slightly different problem.
The discount showed and everything was fine right up until the payment stage. The payment failed (no idea why - it was a 99p payment) but the system now thinks I have already used the code so that when I try to place the same order with a different credit card it will not let me have the discount after all.
So, if your payment fails and you try to purchase again, make sure you double check things before hitting "confirm", in case you encounter the same bug I have.
I emailed support several days ago but have received no response (apart from the "Thank you for contacting Cakewalk" automated email) and I now cannot use the discount code on any product.