nothing spawns on witch hut

my world generated in mcpe last version had witch hut made into witch farm. It has 3 platforms at y lvl 64 67 70(roof).
in version 1.2 only general mobs spawn on 1st floor.
on top and 2nd floor nothing spawns.
so i re-created the same seed to check the witch hut location. It does have the structure so I checked again in the new world. same result. nothing on two platform, only normal mobs on bottom floor.