Nike Defyallday Shoes Falling Apart, Store Claims No Issue

I purchased a pair of Nike Defyallday shoes, and shortly after, the front bottom part started coming apart. I hadn’t worn them much, so I took them to the Nike store as soon as I noticed the issue while it was still small. After waiting 15 days, they told me there was no problem with the shoes. Now, the bottom has separated even more, and the shoes are barely usable. How can they claim there’s no issue when the shoes are clearly defective? I’m extremely frustrated and expect Nike to take responsibility. I either want a replacement or a refund. Nike, please resolve this immediately!