I'm currently making an endermen spawner (a 4 block high room with no light and 3 block thick walls that only endermen can get through by teleporting outside of the room) however it has been a day since I constructed this room and so far no endermen. I'm wondering if there is something I am not doing that is preventing them from spawning, perhaps?
Enderman have less of a chance spawning then other mobs IIRC not only that if they touch water they teleport away. Also they can teleport whenever and wherever, they also teleport sometimes when hit, even if they get killed sometimes they teleport and immediately die. The End which is not implemented yet is the only efficient way to farm them everything else is a crap shoot.
Endermen Spawner