Did not get discount as promoted in store

I purchase 3 jackets at Jet Stores today and I told the lady that I did receive and sms for R75.00 discount, she said to me it will not be applicable as I already qualify for 20% off per jacket. I only manage to look at my slip now ands only to find out that I did not get the discount as promoted in the store. I tried to called them and every time they hang up the phone. It will not be possible for me to return the jackets. I would like them to refund the discount to my account.