Wither Skeleton Farm not Working

I recently finished building my wither skeleton farm and I have the surrounding 128 block slabbed of perimeter but mobs still rarely spawn on the platform. I figured I must be missing a section in my spawn proofing but I turned my render distance up all the way and simply no mobs were spawning. I AFK 20ish blocks below the farm and come up occasionally to see what's spawning but things rarely are. Any ideas what's happening or how to fix it?