I have spent the last several days making a Zombie Pigman Grinder/Gold Farm. this is on Minecraft SMP, and so this took a while. Now it is built, and I cannot get any Zombie pigmen to spawn for the life of me. I have waited at the base/ collection area of the trap for 10 minutes - and none spawned. Here are some screenshots of the trap:
This is the base of the trap, where the Zombie pigmen fall, and I can collect the gold nuggets here.
Looking up at the top of the trap. Basic working principle is that the pigmen spawn up there in the spawning area and walk onto the open gates as if they were solid blocks, then fall 30 blocks to their death.
Spawning Area, from the right side.
Spawning area, from the left side.
I know that they are spawning because there are a lot outside. I'm playing on Normal difficulty.