Whats Wrong With Your Guest Survey Code Redemption? Is It Broken?

Past 2 occasions now I've completed the online guest survey within 48hrs, and I get the voucher code, but when I try to redeem the code within the 30days and only one during that that period, it is rejected as 'invalid'. Is there something wrong with your system. Issuing the codes works but the redemption part is not? Oh yes, and I've tried different branches too. Done another survey there...perhaps 3rd time lucky.