Coupon codes may only be valid on certain products. They will not be valid on products which are already discounted as part of a sale or promotion, this includes items in the Sale Section, Gift Sets, Value Sets & Build Your Own Sets and some site promotions. This may be the reason why your discount is not being applied.
This does not apply to coupon codes issued for Huda's VIPs Rewards, these should be applicable even on sale items (subject to change).
If your cart includes a regular priced product or is a Huda's VIPs Reward code 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 so we can check this for you.