Why won't my mob farm work

I have built a mob farm in my single player world, but it doesn't work.
I have tested the mob farm in a super flat world and it works well. I have built them almost identical, the only difference is that I used snow blocks instead of cobble and that half my mob farm is built in a deep ocean biome and the other half is in an ocean biome. I have checked and there is no light in the farm and no caves under it. I have searched for an asnwer but nothing seems to work.
I copied the farm from logdotzips 34th episode of how to minecraft.
Best Answer
The Problem
The problem that you had is because of the cave underground / near the mob farm. Why superflat world worked? Simply because there is no cave under / below it.
Why?
Why is it? It's because when a mob want to spawn, the game simply search for a random location with certain requirement needed. In this situation is Light Level.
How to fix it
Well simply enough, you must make the light level of near by caves up. You can see the block you're standing light level by entering the Debug Menu by pressing F3. Make sure there is no near by cave to have light level of 7 or below. Make sure also there is no cave with light level 7 or below in 64 block from the middle of the farm.
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Why my mob farm is not working?
Check to see whether it's complete darkness inside the mob spawner or not. 0 means no light's coming through. If the light level is at 8 or higher, mobs will stop spawning.Why is my mob farm not spawning any mobs?
It's over 128 blocks above ground, and the top is spawn proofed. The blocks in the center of each spawning location are to prevent spider spawns.How do you activate a mob farm?
Steps to Use a Monster SpawnerHow close do you have to be to a mob farm for it to work?
In the case of a spawner, players need to be at a distance of at least 16 blocks distance from the spawner to activate it. When AFK-ing at a spawner-based farm, players should be at a distance of less than 16 blocks from the spawner.LICD: Why Won't She Leave
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