Invalid rewards voucher and No help from customer service

African bank was/is giving clients who swipes their card 10 times in a month a R150 checks voucher. I received my voucher in the beginning and all was well. Now in August I swiped 10 times and as usual I got my voucher, only this time the voucher I was given is invalid and I went to checkers 3 times to try and use it and it's not valid. I tried calling and emailing the rewards line but they ignore my mails and the number to call on the sms does not work. They have really disappointed me and for that I will stop using this bank.