Mob spawners have no mobs in them

I found two mob spawners near each other, one was a spider, one was a zombie. I marked it down so I could come back later and make a grinder out of it. I came back a few hours later and the small spinning version of the mob inside is gone and it's not spawning anything. I don't know if this is a bug or something with my world, I hope it's a bug.
And no, I won't go into creative and put a spawn egg in them.