Wither Skeletons won't spawn on Wither Rose

I was building a wither skeleton farm 1 block under the bounding area of the nether fortress with nether bricks. The wither skeletons and other mobs spawned with no problems.

Then i decided to change the floor with netherack plus wither rose and it does not spawn a single wither skeleton.

I then went into a creative world and tried both netherack and dirt. moved the platform multiple times to be more close to the fortress/inside the bounding box but still no wither skeletons want to spawn.

notes: Gamemode hard, other mobs spawn right next to the platform