Mobs won't spawn in the Nether unless you reload the world

I searched for almost an hour through the Nether in multiple worlds, and no Zombie Pigmen, Ghasts, Wither Skeletons, or Magma Cubes have spawned.  The only mobs that spawned were blazes.  The only way to get them to spawn is to quit to title while still in the Nether and reload the world.