Moen Single Handle Shower Faucet-turn on get cold, little hot

I have a Moen Single Handle Shower Faucet, it worked fine when we purchase the new house, two years later if I turn the handle I get alot of cold water coming out, but as the handle goes to the mixed level of hot and cold, I get very little hot water coming out. I have removed the handle and there are two plastic rings with gear like teeth, what do I do to get a better mix of cold and hot water. Help!