Confused

I have a question for Woolworths. You are currently running specials - up to 30% on certain food products. You get 20% automatically, 5% if you swipe your Woolies card and 5% if you pay with your Woolies card. So the 20% and the 5% for swiping your card are allocated correctly, but when I swipe my card to pay the additional 5% (for the particular items) it is not deducted. This has happened to me twice now. Can someone maybe explain to me how this works??