Minimum order value: It's possible that your voucher is only valid starting at a minimum amount. You can find this on the coupon.
Invalid code: Carefully check to see if you have correctly entered the code. You need to look at both the small and large print. Perhaps the code was already redeemed?
Expiration date has passed: Please check to see if the voucher has expired.
Criteria not met: Your voucher may only be valid for a certain part of our range. This information can be found on the coupon. If nothing is specified, you can apply it to our entire range.