Have you checked that you are in the mob spawn area? Because you may have changed it in the settings (it is useful to reduce the spawn area of the mobs because it reduces the lag).
What do you mean by spawn area?
Just to be clear - this isn't a spawner.
I have also tried standing in the area where the mobs should spawn (imagine a 2 block high room 9x9 in size) in complete darkness and no mobs ended up spawning.
Where are these settings that I can manipulate?