Mob spawners not spawning mobs

My friend and I have both a zombie spawner and skeleton spawner.  Occasionally, they'll just stop spawning mobs.  I have stood directly on top of both spawners (in complete darkness) and no spawns will occur.

Steps to Reproduce:

Find a spawner,

Clear 9x9x9 area around the spawner,

Let mobs drop down,

Have no mobs after a while

Observed Results:

No mobs

Expected Results:

Mobs

 

Notes:

This is very annoying, as our Zombie spawner is our main source of XP.  If we restart the world, we get mobs, but until then it spawns 0.