Snap to floor not working with my own floor

I’ve made a static mesh, imported it and am trying to use it as a floor. Snap to floor doesn’t work with it.

If I use the “Floor400x400” mesh included in the starter content, I can snap to the floor. If I load up Minimal_Default and snap to floor on the 2 floors there it works. If I change the floor mesh to my floor mesh, it stops working.

What silly hack do I have to do to get this editor to consider my floor an actual floor?

Keep in mind there should be no difference in collision - just changing the mesh from a 400x400x20 mesh to a 1000x1000x20 mesh breaks this.

To answer my own question - apparently i needed to “build” the level before it would work. Non-obvious.