I built a zombie pigmen farm in the nether, at around y=240, and I can't get any to spawn. The platform is 29 by 29, which is 841 spawn spaces, which should be large enough to get a nice spawn rate. However, I did get a couple to spawn after 10 minutes, and then none for another 15. I made sure I was not standing too close, or playing on peaceful. Another thing is, I checked F3 and saw that there were only 7 entities in the chunk, and 4 were the iron golems that I was looking at.
Also, the floor is made of white stained clay, NOT lava. I know for a fact that they can spawn on other surfaces than netherrack. A ytuber by the name of Ethoslab has a farm EXACTLY like this, and works great.
The farm works by zombie pigmen spawning, iron golems killing them, and their drops being collected by a hopper minecart that travels underneath the platforn. I need HELP! PS: Does it have to do with LC values or something like that?