Dischem arrogance in.customer service astounding.

So, dischem in September you sent me a birthday voucher for R100, valid for 12 months. Today, I had some treatments and wanted to redeem the voucher, unfortunately, when I got to the till the voucher could not be redeemed due to system changes. The voucher was part of my budget. So, explain to me why your system changes should inconvenience me?Why is it that this was highlighted only once I got to the till. Rather pathetic customer service.Why, could this not be rectified immediately?