Discounts are set up to start and end on specific dates. If you have a discount that was once working but is no longer working, make sure the discount is still active. If it isn't, edit the discount and expand the expiration date for the discount.
To view discounts, go to Configure | Discounts, click Show,
and select Show Coupons (if you are looking for coupons) and Show InActive Discounts to see discounts that have expired. If you find a discount that has expired, edit the end date for the discount, and the discount will work again.