New device not working and the store refuse to ex

I bought a samsung tab 10.1 for my daughter at the end of January 2015 for using it at school. It has been a nightmare since then and it's even affecting her school work as it doesn't work 99 percent of the time. All her school books are on this device she can't even do homework because it's acting up. We took it to Samsung the Glen mall twice and it came back with the same issues. This is annoying. I spent so much money on this thinking I was buying the best for my child's education not knowing I was making a big mistake. Why can't you exchange this device since you fail to fix it.its irritating.