Brand match is a false marketing strategy

On the 16/03/16 I went to Pick n pay Tramshed to buy crockery, after paying I got a promotional discount voucher of R18.39,i was impressed thinking that brand match is finally working for me since I only saw it advertised, I was shocked on the 18/03/16 when I tried to use that voucher it says R0.00 discount even though I never use it and it was suppose to expire on the 30/03/16.I am still don't know what happened even the cashier failed to explain what was wrong, hence I am saying its a false marketing strategy.