Wither Skeletons Not Spawning

After spending >8 hours inside of a Nether Fortress partially overlapping with a Soul Sand Valley on Hard Mode, 0 Wither Skeletons have spawned, but countless skeletons have. Normally I would attribute anything of that sort to bad luck, but due to the number of hours and the consistent spawning of skeletons and other mobs, it seems unlikely to only be misfortune.