NPC Models floating above the floor or a larger NPCs model is partially in the floor

Currently, I am trying to make an intimidating boss and a miniboss to go along with him for my game, but the skeleton models keeps phasing through the floor, and they are larger (miniboss: size 2, boss: size 4). Also, many of the smaller skeleton NPCs are either perfectly on the dungeon ground, or floating a couple inchest above it and I can't see a way to fix it since there seems to be no obvious hitbox editor (unless I'm wrong).

Thank you!