Pathetic aftersales service

So I order KFC, and try to use my FNB voucher which only expires on 31 December. I load the voucher AND make sure it is selected before ordering, and they still charge me full price for my meal. After messaging and letting them know that my voucher didn't load correctly I get the following lame reply: 'We are sorry to hear about the inconvenience. We have looked into this for you and can see that this promo code was set up for a specific group of people. Only these customers are able to use it. As a result, we won’t be able to adjust the amount of your last order.'

So please tell me WHAT is the specific group of people? I am an FNB member with an FNB voucher code ?!?!?!?! And I was allowed to load the voucher and select it. Pathetic!, Have deleted the app and won't be using again.