This bug is of how the wither skeletons spawn recently. In my world I have a wither skeleton farm. It used to be highly productive. Since the Minecraft 14w30c snapshot, I noticed 0 wither skeletons were spawning in the farm. Zombie pigmen, magma cubes, blazes (which only naturally spawn in nether fortresses), and even skeletons (20% chance of spawning in place of wither skeleton) were spawning in this place. I have several of these 10 high setups around the fortress. The place where the mobs spawn is 2.5 blocks tall.
I'm not too sure when this bug began. I believe it probably has to do with some changing in the height required to spawn wither skeletons. Skeletons do spawn which is weird. I built these setups at intersections by the way.
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To clarify, Wither Skeletons are spawning in 3-high spaces—just not in 2.5-high spaces, as they had in the past. It's possible this is an intentional change, though I'm hoping it gets reverted. (Running 1.8-pre1.)
Confirmed for 32b. I've got a same kind of farm which used to work in the past. I walked away 128 metres about 15 times, and after returning not a single wither skeleton had spawned. Blazes, magma cubes, zombie pigman and regular skeletons did.