There's a good chance that your mobs aren't far enough away from the spawner for it to pass the check needed to spawn another entity.
Gee, this a popular discusion. Oh, and i remeber once i was in a ravine, no lava, and it was ultra dark. But there were no mobs! I logged out of Empire Minecraft and spent some time watching Youtube. Then, when i logged back in, it was filled with mobs.