Me and my friend have recently built ilmango his mob farm. Now I have tested this mob farm in my single player world and it spawned a ton of mobs as promised. Now the thing is we built this mob farm on a Realm. I started thinking it maybe had to do something with the fact it is on a Realm? We used the exact same dimensions as ilmango and we have the same y levels etc.
So yeah, we play on a Minecraft realm.
The difficulty is set to Normal, maybe this has something to do with the rates aswell?