Two players must be in nether for spawns to happen

Hey I have a question for my Spiggot server, it seems that my pigman farm, wither skeleton farm, and magma cube farm only works when two people are in the nether, instead of only having one. I was told this might have to do with spigot but have searched for a while to no avail.
Plugins:

• CoreProtect
• dynmap
• Invsee

Any and all help would be appreciated

Also yes I looked at the article editing the bukkit.yml and spigot.yml files too.