Installing NPC's to a server not working?

There is an NPC addon I put in my server, I followed the standard addon format, made a folder for the addon in /addons and put the stuff in there, however the NPC’s that the addon installs wont work on the server, whenever I try and spawn them I just get: “Sorry you can’t spawn that NPC!” I’ve tried installing another version of the addon to the server to see if it was just the addon, but that didn’t work either. They both work in single player. Any ideas?