Checkers Voucher not valid continusly

My wife has the Checkers App on her cell phone and has won several vouchers wereby when wanting to claim in store the cashier advises the voucher number is not valid. She called in 4 times already to raise the concern whereby each time she is advised they will escalate the issue and will call back. This is now pending for several weeks with no feedback. When she called again today the same situation took place and they advised they will escalate and call back. Why do checkers issue discount vouchers etc if this is the service received if the voucher is not able to be claimed.