Gift Card Discount Disappears on Checkout

In testing I’ve found that when initially entered into the “apply coupon” field (I don’t see a filed for gift card), in the cart the code is applied and the correct amount is deducted from the order. However, when I move to the checkout page, the code/discount appears for a second and then disappears. When I re-enter it again a message is returned saying the “code does not exist.”