I'm trying to figure out how to make only Zombies spawn in a specific area in my Minecraft server's spawn. I have a pit in my spawn that I would like to have spawn Zombies each night time cycle. The idea is to allow new players to kill them for XP and just for fun. I have WorldGuard and SilkSpawners. I've tried setting up a region within the pit but no Zombies will spawn. I tried placing a Zombie spawner but again, no Zombies will spawn. The entire spawn is a region (region named spawn) and the pit (spawn-pit) is inside that region.
mob-spawning isn't allowed in the spawn region and all hostile mobs are flagged in deny-spawn, including Zombies. I'm wondering if the overlap of the two regions with conflicting flags is the issue and how to fix it. Ideally I would also like around 10-15 Zombies to spawn in the pit during the night cycle. Any help would be greatly appreciated.