Ryans theory was the spring was not pushing hard enough on the ratchet that it was not engaging. If your teeth are worn off or severely rounded the stronger spring will not be a fix. You would need to purchase a new mechanism.
I agree with Tony. You have to check the teeth on the kick starter mechanism FIRST to make sure they aren't worn. If they are, that is your problem. In the case that I had with mine, teeth showed no sign of wear. So, after installing the strong spring, the slipping teeth problem went away.