If you use mortised posts and the slope is severe you often find you can't stand the post upright. The shaped rail will pinch in the mortise. To overcome this I generally shave the end rail a ittle more - either with an adze or saw,
You can also just use another bag of postcrete. Throw some water in first, then the postscrete and more water. It will set like a bullet and bind to the old stuff.
