Answer
Some codes can be used by many shoppers, while others are unique, single-use or limited to one redemption per account or customer. The retailer controls the redemption rule.
If a code worked previously but is now rejected, it may have reached a usage limit, been redeemed already or been withdrawn. Check the current retailer terms rather than assuming the code remains reusable.
What to check before checkout
Read the retailer’s current promotion terms, confirm that the code applies to the items in your basket, and make sure the expected saving appears before you pay.