There is something I keep missing, I built a mobfarm (dark cube with water/dispensers and a 30m drop). This thing is about 200m in the sky above the sea, so there is no cave no mountain or anything within a 128m radius.
I keep waiting for them to spawn about 30m away (btw I play on normal mode).
Around my house on the ground mobs keep spawning as I am used to it.
I hope you may have a idea what I did wrong, I searched the internet all day and I could not find any solution.