Is this survival? Factions? Did you check any lighting that would decrease the spawn count? Is the area too enclosed for any mobs to spawn? Check all the things that you are in control of. If everything is fine, space, lighting, etc., THEN ask about the server's settings. Before I made a grinder and it gave me about 5 mobs per hour. I realized that I enclosed the area too much, so check all those things first.