How do I get tons of mob spawns on my server?

Recently I created a staff server for of course my staff and I. Its a skyblock server where we only have one island that we all live on. But when I made a mob drop/grinder there seems to be about 1 mob dropping every 3 minutes and it's painful to watch. I took all the necessary actions such as putting it 24+ blocks away from the world spawn, I've tried changing the spawn limits in bukkit.yml and in worldguard. I've also tried that MobManager and MobspawnSettings and they do not work what so ever. I've tried changing my difficulty to all of them except for Peaceful of course. But none of them change the spawn limits. I've googled it many times looking for an answer but there seems to be nothing thats working. The skyblock world is a fully empty world other than our island so I don't find there a reason to why the mob spawning is so horrible. Any help would be appreciated, thank you.

 

First time I tried to make a mob grinder from spawner in creative it didn't work, that was until I placed the spawner in an area with mossy cobblestone on the floor and walls. After that the mobs flowed thick and fast.

 

Well it's a survival skyblock server so its impossible for me to get a spawner.. Aha.