pigman zombie spawn very slow in nether.

What happens when you run kill all entities(running /kill @e)? You might be reaching the mob cap

I'm having the same issue, I think. I started building a pigman spawner above the bedrock layer, but nothing has spawned on it at all. I killed all entities (which also destroyed all my decorative item frames and armour stands in the overworld, so thats a little annoying) and it didn't change anything. I'm playing in 18w31a