Mobs not spawning

Mobs are not spawning in my mob tower. I was AFK for 2 hours and only a zombie and a creeper spawned.
My mob tower: http://www.youtube.com/watch?v=3S5x3qxl8s8
I was wondering if this was the bug with the way airblocks are treated differently tha docm77 mentioned in http://www.youtube.com/watch?v=pz7Iu7L1ZiQ&feature=c4-overview&list=UU4O9HKe9Jt5yAhKuNv3LXpQ but I thought that was only in the snapshots.

Edit: Spawn rates have gone back up now I may have accidently been using the snapshot.