Why give customers invalid vouchers?!?!

I added thing into a cart and had issues with paying with my new bank card. I left the order half way and then cotton on decided to give me 10% voucher on the order..however when I used the voucher number it kept saying "voucher number is invalid". Why send customers vouchers that do not work??? I've always had respect for this brand but after this incident, it has left a bitter taste in my mouth. I would have ordered irresrespective of the voucher.