There are a few possible reasons if code not be able to use;
1) There are no available store codes for the purchase
2) the codes have exceeded its maximum usage
3) The codes are recently expired
4) The codes do not meet the terms and conditions (Displayed in the Coupon Section).
Nevertheless, ShopBack always takes pride in aggregating daily promotional and coupon codes that are eligible for Cashback.
Please contact our friendly HappyBag bot here if you have any further concerns and we will be at your assistance.