Help. I need help with making zombie npcs.

Hey guys. I’ve been trying to make a Feral Ghoul zombie NPC and I use the class npc_fastzombie/npc_zombie. It doesnt change the model. All it does is make another zombie and the name is different and thats it. I use npc_citizen and it works also combine does too. Does someone now how to fix this? Or can you help me in teamviewer?