I'm building my first witch farm in creative and can't get anything to spawn in it. Not witches, nor anything else. So I'm sure I've got it at least roughly in the right spot.
1) I found a hut, and using this help page at MCW I started building my farm. I've actually got 3 platforms with 2 blocks between, as witches have a spawn height of 1.95 (the tut videos in there show 3 platforms as well). The volume it said is valid is 2 blocks below the floor, the maximum reach of the porch, and 1 block above the roof. I figure, even if I'm off by a block up or down, the central floor should still spawn witches and maybe the upper or lower floor.
2) I set a few command blocks to fill the ground around me with glowstone. So starting at a minimum radius of 100 and going out to 150+, I had some command blocks target me with the fill command to fill in from 1-70 with glowstone. Outside of that I have a bunch of glowstone randomly placed kinda lighting the place up on the surface. Inside of that most of the caves are filled using similar commands or are torched up.
3) My total entity count varies from ~5 up to ~20, with ~20 occurring if I fly around. Occasionally I run 'kill @e[type=!player] to see what's spawning, and I'll get 4-5 hostile mobs, well below the cap of 70 hostile mobs. I usually float less than 5 entities spawned in.
4) I'm sitting at a distance of about 25 blocks away from the outside edge of the farm. Too far for mobs to walk around, but they should still spawn.
5) I've got a command block that kills anything outside of a certain radius of the farm. I verified that witches can spawn and not be killed by the command block by spawning one in on one of the platforms.
6) I've verified light levels. 0. I've got a ceiling 3 block above the farm going out 30 blocks in all directions. Overkill, but I wanted to be sure. It's completely covered in torches. Nothing spawns on it.
7) I've got day/night turned off with it froze with a new moon at peak.
8) The seed I'm using: -5629280954827210877. Location of the hut: 514/-265. If you do check it out, the levels of the platforms that I've got are 70, 67 and 64. NW corner is 512/-272 and SE corner is 518/-264. The place I'm AFKing at is 515/-299.
9) I just tried removing the top level of the farm to see if maybe the height space of 2 was stopping it. Nothing so far. I also tried, with the top floor removed, to move up the central floor. So there's two platforms with at least 3 blocks of height to spawn stuff in. Still nothing. I've got a platform down at Y59, 4 or 5 blocks below the bottom floor of the witch farm. Nothing spawns there either (other mobs, I mean).