general problems of the 2010 Nissan Murano

I purchased 4 michelin tires from sears in July of 2013. In September of 2014 I took my car for its normal oil change and my mechanic told me I needed new tires. I looked at him strangely and told him I just purchased the ties last year. He then instructed me to go back to where I purchased the tires and tell them that the tires were defective because chunks of the treads were separating from the tires. I took my tires to sears and upon inspection they decided that the tires were defective and they replaced only two tires. I explained to them that I believed there were problems with all four and I wanted all tires changed but they decided to only change two. Here we are in November of 2015 and the sam issue is happening again. I called michelin and they told me there are no recalls on the tires but I find it hard to believe that this is not happening to others. Also when I asked them to provide me with somewhere I can file a complaint they refused to give me any information.