Hostile mobs wont spawn

I loaded the latest prereleases 6 and 7 for the new update. loaded up a world, and built a mobfarm in creative mode. I then set it to night and tested it. nothing had fallen after 10 mins, i then fell to the ground and nothing spawned. checked world was on normal, mob spawning command was true and I did the difficulty command (still with no hostile mobs spawning). The world I was in is attached in a Winrar archive.