Blaze, wither skeletons and magma cubes won't spawn in nether fortresses

when i go to the nether fortress, i find 2-3 wither skeletons, blaze spawns only from the spawner, magma cubes are nowhere to be seen. i need wither skulls, but, from the few wither skeletons I've killed, no drops. i have farm which was producing tons of mobs of every kind, now, in doesn't have one