I made a mob farm to use for xp, but it barely worked during the day and kinda worked during the night. So I made it again in the middle of an ocean, but that one didn't work at all. Then I went to YouTube to find a working xp/mob farm. The one I found was for 1.16 but that doesn't explain why it shouldn't work for a different version of Minecraft. When they were on hard mode in the tutorial after it was made they had TONS of mobs coming down the entire time, I made it exactly the same and was locked on hard mode and after 1 hour I only got a zombie and a spider. That was when I realized that it wasn't me just being stupid or bad at making mob farms.