Why is it whenever I use unions this happens

I was trying to use union but whenever I make one I makes a weird invisible wall.

video of problem
robloxapp-20200909-1131409.wmv (1.9 MB)


When you unioned it, it got rid of a certain area of the part and made the end transparent but not collidable.
To fix that make the part that is unioning it longer.

Whenever you union something, Roblox will generate a simplified collision model to save space. That means large hallways might have invisible slopes or walls.

You can set your union’s CollisionFidelity (whatever it’s called) to PreciseConvexDecomposition, or just don’t union it at all. In fact it’s best to use unions in moderation - don’t union everything you build.