Put the summary of the bug you're having here

What I expected to happen was...:

Mobs should be spawning day and night because of the overhang of the roof and the light level is low.

What actually happened was...:

I followed a video on YouTube of how to make a Creeper farm and all of the specs are correct but no mobs are spawning. This is the second XP farm I made and no mobs have spawned in them.

Steps to Reproduce:
1. Build a 6 layer mob spawner 17 long and 17 wide
2. Add trap doors on the ceiling 
3. wait and see no mobs spawn