Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Doesn't work!!!

I bought this collar for my dog because I just moved into a house with 5 people and he keeps barking at them! I put it on him for the first time and took him outside to try it out. He started barking at my neighbor and nothing happened. He barked about five times and then all of a sudden he cried and jumped straight into the air. I guess it had worked. However, that's the ONLY time! I have had it on him for a week and he will start barking and I will wait for the collar to work, and eventually have to quiet him myself since I don't want him to continue annoying people.I contacted Amazon to return the product since this was a COMPLETE waste of money, and they won't let me since I opened the packaging!!! I am going to continue in my fight, but I wanted to warn you to NOT BUY THIS COLLAR!!!