Sometimes spawners will not spawn mobs under certain circumstances. I have been unable to discern exactly which factors cause this behavior, but I am able to reproduce this issue by doing the following:

1. Start dedicated server with world with seed -4282704775408294980 and difficulty not peaceful.
2. Teleport to location 121 28 -158, where there is a naturally generated spawner in a dungeon.
3. No mobs spawn from the spawner.

Spawners will still spawn mobs in other cases. For example, loading the exact same world in single player will cause mobs to spawn from the spawner.