Why can't they make the 20% discount that it pulled through automatically

I have utilize my vouchers that I got now from lockdown, and one of my college asked me if I got the 20% discount as well I said no it didn't show when I checked out the flight tickets. After I went to the voucher I see their is a 20% discount. Now I have lost almost R2000 on the voucher. My Question to you is WHY could they not load the extra 20% on the voucher so if you check out it will pull through. I have phoned FlySafair to rectify and they said it is water under the bridge. Their is so many things we went through over the COVID period and now this as well. Why couldn't you make it automatic 20 % discount when you fill in the voucher. I am really disappointed in this because this is now the second time that I have tried to get rectified but every time it is the consumer that suffers.