Low Mob Spawning

[Gweppy]

So my friend and I recently started a server with two other of our other friends.  Game was set to easy difficulty and at the start, we hardly had any hostile mobs spawning.  we didn’t think anything of it. Later though, I built a mob grinder high above the surface and have hardly any mobs spawn . Please help us fix this.

[SWOEN]

Set the difficulty to "normal" or "hard" to increase mob spawns