Disappointed with Truworths

I purchased a jacket from Truworths Pavillion which was on promotion, they did not have my size so I took a size bigger because I did not want to loose out on the promotion. I thought I would have been able to exchange it at a store that had the correct size. I called a few stores and found my size at Musgrave center. Yesterday I went to fetch the correct size from the Musgrave store. When I purchased it from Pavilion I was given a 33% discount however when I changed the size yesterday I was not given the same discount. I was only changing the size not taking a different item so I cannot understand why the discount could not apply. The store assistant was helpful and tried to call head office but head office refused to give me the discount even though I was only changing the size.