broken wither

I tried to make the wither one day in minecraft super flat just for fun. When I put down the three heads on the 4 soul sand, nothing happened! I thought it might be a problem with my laptop, so I called my cousin. I have a macbook and he has a dell. He said it didn't work for him, either! I think it is a problem with the update, because I've done it many times before. I even went on youtube and the forums to make sure I was doing it right. I was. I tried it for the last time, AND MY MINECRAFT CRASHED!!!!