Coupon codes may only be valid on certain products, and they will not be valid on products which are already discounted as part of a sale or promotion, this includes items in the 'GoodBye, Good Buy' sale section. This may be the reason why your discount is not being applied.
If your cart includes a regular priced product and cannot be redeemed please check the code has been entered correctly then send us a screenshot of the error you have received and details of your code.