Nether Structure help

I have searched through forums retaining to this issues and it says to make the biome an underground biome when spawning in nether/nether-like dimension. I do this, but it spawns inside the walls/ceiling. When I limit the structure to spawning on the biomes grass block, the structure flatout doesn't spawn at all. Is this meant to happen, and is there a way to combat this? (I've attempted offsets but that just leads to loads of half visible structures floating about)