Redeem voucher that does not exist for Vitality

I joined the Discovery Vitality programme and they gave me a redeemable voucher of R500 that I could use at any Pick n Pay store. When I got to the till I followed the instructions of how to redeem the voucher but every time they entered the voucher code it declined. They tried atleast 8 times. I was so embarrassed because there was long queue plus worst of all was that I budgeted on that R500 voucher which through me out of pocket. So why do we get these rewards that do not work and I firmly believe that I need to get to the bottom of this because by right i was given a voucher and if it's declining which means it must of being used which is *****. From a customer service perspective this is pathetic!