Why Am I Not Getting 40% Off With My iRewards Card?

I have an iRewards card that I use for my local Chapters. I am supposed to get 10% all books in store, and 40% off all bestsellers. I have recently bought the first three twilight books. When I bought Twilight it says “new york times bestseller” and I only got 10% off (its a bestseller shouldnt I be getting 40% off?) then I bought the 2nd book New Moon and its also a new york times best seller and I still only got 10% off. Then I went and got the 3rd book Eclipse and its a #1 new york times best seller, a #1 USA best seller, and a #1 Wall Street Journal bestseller. Still only got 10% off! My mum was outside waiting in the car and shes very impatient. I didnt have time to ask why I wasnt getting the right discount. I lost my card somehow so everytime I have been getting them to just type my name into the computer and use it that way. As I said my mum is impatient and I didnt have time to issue a new card. Why am I not getting 40% off these books??!!