Why isn't my Minecraft wither skeleton farm spawning any wither skeletons?
Replace the stone brick slabs with full blocks and make sure that your landing pads definately have 3-4 blocks of open space above them.
Also wither skeletons have specific light conditions requiring them to spawn the wiki page is linked below.
- Wither skeletons will only spawn near Nether fortress in light levels of 7 or less.
- Wither Skeletons can spawn in a 2.5 block tall space
http://minecraft.gamepedia.com/Wither_Skeleton#Spawning
Wither skeletons are a rare spawn so don't expect extremely high rates but the above should sort out your spawning issues.