Wither Skeletons and Blazes not spawning in fortresses(besides spawner, obviously)

In our smp world the fortresses we generated in minecraft 1.7 have stopped spawning wither skeletons and blazes, they have been thoroughly search and the number of zombie pigmen in the fortress only grows until the point where the mobs could not spawn any way. We have tried reloading the chunks and this process has been demonstrated numerous times. The fortresses are not modified heavily and resemble their raw structure.