Check the fine print for any exclusions or restrictions on the coupon code. Ensure the code hasn't expired and that it applies to the items in the cart.
Try a different web browser or device to see if the code works there. Disable browser extensions temporarily, as they might interfere with the code. Clear cookies and site data to give the browser a fresh start.
Use incognito or private browsing mode to avoid any cached data issues. If none of these steps work, contact customer service for assistance or search for a new working promo code.