There are two types of codes that can be used in the Cart Page. The most common reason is the code is being entered in the wrong section.
1) A promo card should go in the Discount Codes section/tab
2) A purchased gift card should go in the Gift Card section/tab.
To know which one to use, Promo Gift Cards usually have expiry dates, while Purchased gift cards never expire.
Once you enter the code, hit the "Apply/Activate" button to apply your card. If this still does not work, please let us know.