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.
Best Answer
The answer you are looking for is hidden inside you own question: "there are a lot outside". The game randomly checks places to put mobs and since the outside is much larger than your trap, most of the zombie pigmen spawn there. The mob limit is exhausted pretty much instantly and the game doesn't spawn any more, unless some of the existing mobs de-spawn or get killed.
You will get zombie pigmen to spawn in your trap eventually, but you may have to wait hours for just a few spawns. You need to prevent spawning outside your trap to improve rates of your trap. This is very difficult to do in the nether, since you can't use light or water to prevent spawning. That is why you don't see zombie pigmen traps, building one is a monumental project. Notice for example here a huge area is covered with half-slabs to prevent spawning.
The spawnable area is +/- 8 chunks from each chunk where there is a player (minus the chunks on the edges of that area). If a single player is online, mobs spawn in 240x240 blocks area centered on the player chunk, that is roughly +/- 120 blocks from the player.