poor nether mob spawns

Poor mob spawns..
We biult a gold farm. Not realizing the poor mob spawns in the nether on our realm survival world. We expected easy gold, but little or no mobs spawned at all! it only seemed to occur on the realm. spawn rates multiplyed ( especially pigmen) in singleplayer mode.Im not sure whether this is a realm setting or not, but i'd like to have lots of mobs in the nether! make it fun!