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.