Blocks don't spawn on air

I'm making a cloud-like block, this block is very important for my mod since the item it drops serves for getting to the end of the game.

Blocks don't seem to be able to spawn replacing air. I put the spawning Y from 99 to 141 replacing air, avarage of 1 per chunk and average of 17 per group.

They spawn perfectly fine if I want to replace any other block that isn't air, but I specifically need these to be air. No, I don't wanna make them spawn as a structure, I want them to spawn naturally in semi-random forms just like any other modded block would do.