I recently built a zombie pigmen farm underground in the overworld. The design is as depicted here: 5 maximum sized nether portals next to each other with canals transporting the spawning pigmen to a falling damage trap.

However my nether portals don't spawn a single zombie pigman in the overworld. So here's the question, why?

• MC version is 1.8 on a local server.
• The portals are shared with another portal that we use as adventuring portal. However I never witnessed something spawning there, either.
• We use a bunch of mods, of which these may be relevant to spawning: Mo' Creatures, Custom Mob Spawner, Infernal Mobs, Animals Plus

My best guess is the shared portal thing as no mod seems to interfere with anything concerning portals. Do you have any ideas?

edit: The full mods list

• [1.8] ChickenShed (v1.2.0)
• [1.8]bspkrsCore-universal-7.01
• [1.8]Treecapitator-universal-2.0.5
• 1.8-MB_Battlegear2-Bullseye-1.0.10.1
• AnimalsPlus-1.6
• antiqueatlas-1.8-4.2.10
• AutoSap-MC1.8-1.2
• CustomMobSpawner 3.4.0
• DrZhark's Mo'Creatures Mod-8.0.1
• dungeonpack-1.8
• emeraldobsidianmod-1.8-1.3.1
• InfernalMobs-1.8
• SmeltCycle-2.0
• TheLizzyAnvil-1.8-1.0
• VeinMiner-1.8_0.30.2.unknown

I don't remember in which snapshot this changed, but I just checked again in 1.9.2. That gives one more option for corralling those uncooperative villagers.