What i do when i'm testing a coupon if it is working:
I try to reg a garbage domain, apply the coupon in the shopping cart to see if the discount works.
If the coupon works, i add my real domain which is now at regular price. I then cancel the garbage domain and the coupon discount will now apply to my real domain. Then i process the order.
This way, if the coupon did not work in the first place, the system will not know the real domain i'm trying to reg. lol