Wither Not Spawning

So to get this bug, generate a new world using Minecraft version 1.10.2, then go into the world and make a get 4 soul sand and 3 wither heads. Next attempt to spawn in the wither by building the wither spawning structure using the soul sand and wither heads. Nothing will happen now and the structure will not transform into the wither we know an love. Note, I am not in peaceful and even when using the /summon command, I cannot spawn in a wither or even a giant.