Double-check the coupon's expiration date and ensure it hasn't expired. Verify the coupon's terms and conditions to confirm it applies to the items in the cart. Ensure the minimum purchase requirements are met if applicable. Check for any typos in the coupon code and re-enter it carefully. Confirm that no other coupons are already applied to the order, as some sites only allow one coupon per purchase. Contact customer service for assistance if the issue persists.