Whenever I stand in this certain location, zombies just pile up and not go around my debris. Anyway to fix this?

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I've always assumed that using cylinders for clipping is a bad idea. Try clipping that area with regular brushes instead.

similar issue except its when i jump onto a car they all stop dead...

on the kvps of the clips, there should be something called navmesh. Check that and it'll make the zombies navigate around it instead of trying to go straight through it. i love hand sanitizer.