Make sure you're not shaking too much. It sounds weird but it used to happen to me too. Your feet have to be ready to slide the rail steadily, if they're limp they will do everything you don't want them to do. I overcame this by just starting off with an easier rail and just getting more comfortable sliding. And also just think of every rail as the same. Don't let your mind sike you out and make you think something is harder than it really is. I mean the pros throw the fuck down on the craziest shit so we all should be physically able to do crazy things too, right?
Sunday River and Sugarloaf USA