Wither Skeletons and Blazes Do Not Spawn Again in Nether Fortresses

Mobs spawned with 1st initialized THe Nether... updated to 1.2.10 and my pre-generated world no longer spawns neither Wither Skeletons, blazes, magma cubes... only pigmen and ghasts. I know this issue was previously resolved... please check if this is related to the nether brick somehow. Its happening again after the latest update.