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.