It could be that you added too much powdered sugar. When I first made fondant from scratch I put way to much po.sugar and the next day it was impossible to work with. It also depends on the environment that you live, I just recently moved to a more humid climate and have noticed that I have to add more po. sugar to my recipe than when I lived in a drier climate. Also try kneading in some shortening to see if that would help some.