Promotion but out of stock

There was a promotional ad on tv promoting pampers jumbo pack nappies for R169,99 at Dischem. When I went to Dischem Greenstone on Tuesday 27th October there were no size 5 & size 6 pampers nappies, how do you run a promotion and then stores do not have stock. I emailed Dischem customer care and they told me to go back next day and they still didn't have the sizes I needed and then said when they get in stock they will call me but I needed nappies then!!