Mobs dont float next to walls as in: Mob spawner killer

I tryed to make a mob spawner killer to kill zombies for me but it doesnt work.
I tryed Both game modes but i found that in a 3x3 room or a 5x5 room the mobs will not float unless in the center of the room.
I filled a room up with Creepers and only 1 out of 20 Floated approx.

I tryed to get Villagers from 1 point to another in the sky and that didnt work either.

Water i think is broken like a friend of mine told me.

No i did not build nothing wrong, his Mob Spawner killer is broken and that was from Beta.