I built this 2 layer mob spawner trappy thing in the air. It's 24×24 but no mobs spawn inside 🙁 No one falls down and I've opened it up several times to check; nothing inside either.
It's a standard thingy with 4 water flows, forcing mobs to the middle and drop down. As said I've opened it and nothing inside
It's dark, 100% sealed with stone.
Best Answer
If you are 24 meters away from the spawning floors (or at least a good portion of them), and standing on the surface of a normal world, your problem is most likely the world itself.
You see, the spawning algorithm just tries to shove mobs all over the place, shooting at absolutely random points in a great territory around you. Well, most of this territory can be ignored, as any hostile mob farther that 128 meters from you immediately despawns, but a 128 meter sphere (minus inner 24-meter cutoff) is still large.
So when you add a single dark room within that, but stand atop of sixty meters of cave-filled landmass, a chance for a mob to hit your room is proportional to your room compared to mass of caves, plus all surface at night.
The solution is to prevent outside spawning somehow. Lighting all the caves is doable, but a huge work, so the easiest way is to take to the skies. Build an orbital lift or something, and place your spawner somewhere at y=192, to be 128 meters from sea level. I prefer to build over an ocean, which allows me to build twenty-something meters lower and still not hit undersea caves.