NPC AI (NPCs Float?)

Hello! I'm really struggling to get NPC AI working while whiteboxing my level. The NPCs don't seem to collide with anything, and also don't seem to have gravity--I've made a big gap in between the main walkway and the areas the NPCs are (so they can shoot at the player) but they can just run right across that gap without falling down. I'm using the Skeleton Marksman, if that helps. I've looked up video tutorials but nothing seems to tell me why my NPCs aren't colliding and why they can float. Any help?