Double-check the coupon's expiration date and scope of application. Ensure the coupon complies with the usage rules. Check with the website's customer service to verify the coupon code's validity. The website may have changed or canceled some promotions. Confirm the coupon's source to ensure it is not from an unrelated website. Verify compatibility with the website's coupon function. If the problem persists, report the issue to the website's support team.