I've built a mob spawning trap but mobs are not spawning.
It's a tower that is 10 blocks long x 5 blocks deep x 35 blocks high. At the top of the tower (33 blocks) is a spawn floor. The spawn floor is 10 blocks long x 3 blocks wide. There are signs on the edges of the spawn floor.
I've used a mapper to find all the caves in the immediate area and they are all lit.
When I'm standing on the ground (35 blks below the spawn floor) nothing will spawn.
The game is set on normal, single player, and mobs will at night on the ground, just never in my trap.
Here is a pic of spawn floor (torch added for pic, it is completely dark normally)
Here is a pic of the whole structure (tower is built in the cliff).
I'm not sure what I am doing wrong. Please help.