H&M Denied My 15% Discount on an Exchange After Accepting It Initially

I traded in a bag full of clothes at H&M for a 15% discount, which I used to make a purchase. However, the items I bought were too small, so I returned them, and H&M re-applied my 15% discount for a new purchase. But when I went to the Zorlu H&M store, they refused to honor the same process and denied my discount. This is completely unfair and inconsistent! How can one store apply the policy while another refuses? I demand that H&M restore my 15% discount immediately. Customers shouldn’t have to fight for a promotion they were rightfully given. If H&M advertises a deal, they should honor it at all locations!