One of the below scenarios may be the reason!
Is there already a promo or offer being applied to your order? (these often automatically populate when your order qualifies for an offer) - there is a limit of one discount code per order.
If your code is for a specific for a free item - Please make sure you've added the product to your cart first unless the offer specifies otherwise. (this one gets most people, it's not just you!)
Are you setting up an automatic/subscription delivery? Additional discounts aren’t always valid with subscription items but may be applicable with other 1 off items in your purchase -please reach out to us so we can help out.
Please note, If this doesn't answer your question, it is important that you email us right away to make sure we don't ship your order without helping you out first!