So! I've made a giant mob spawner/killer up in the sky (y=220-235) over an ocean. Light level 1 spawn floors, 540 block area, 3 high, periodic water flush that pushes mobs into a pit, where fall damage kills them.
Problem is, mobs seems to rarely spawn inside, and endermen just vanish. I know they teleport on contact with water, but shouldn't they appear outside the trap on one of the catwalks? The floor they land on is ~40 blocks lower, and there's nothing else up that high.
So my question(s) are, why aren't more monsters spawning? I'll leave it running for half an hour, and there will be 2 spawns. Other times, I'll get 30 kills in 10 minutes. Any idea why?
Bonus round: Where the hell are the endermen going? Why do they hate my trap?
Best Answer
Mob traps built close to the sky limit have been known to end up not being completely loaded by the engine in some situations. The way to test for this is to open the F3 screen and look at the 'lc' value. This number is the highest Y-level that is currently loaded. If it's lower than the top of your mob trap then you would need to add some blocks above the trap to force the entirety of it to be loaded. This youtube video shows how you can fix the problem if it happens
It's also worth mentioning that the high altitude mob traps are (all other factors being equal) always going to be at least somewhat slower than low altitude mob traps simply because there ends up being more blocks for the spawning algorithm to check