Problems with 2010 Nissan Murano TIRES

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.