Skeleton spawner not spawning!

So I started a new world in I believe the snapshot right before the 1.9 pre-release 1 and found a skeleton spawner without a roof in the desert. Now 1.9 pre-release 2, I started to mess with it and enclosed it (No light what so ever) but there aren't any skeletons spawning at all. I dug out a 9x9x6 room so they can spawn but nothing is happening..