Why won't Zombie Pigmen Spawn in my mob trap?

Solution 1:

There seem to be lights around the edge.

For a successful mob farm:

• Valid spawning spaces (solid blocks).

• Dark, 9 or less.

• You must be between 16 and 32 blocks away.

• No other spaces for mobs to spawn.

I have a gold farm that works a little differently to yours, here is a rough plan:

This is on the roof of the nether, so there are no other spawns up there. This increases efficiency.

Solution 2:

Lowering the difficulty in game slows down the spawning (I watch Ethoslab too so I know that he plays on Hard all the time). And make sure your difficulty is not at Easy.

Although it is said that light levels do not affect the spawning of zombie pigmen, copy Etho's design anyways with the glowstone so you can compare between the two for any differences:

And this is pretty obvious but mistakes happen. Make sure it is not on Peaceful (although you have said that you do see spawns happen).

Otherwise, there should not be a problem here. Can you post a picture with the F3 information? Thanks.