Imported Objects falling through imported terrain and non-imported terrain

I have some objects imported from Blender, but when I play the scene, the objects fall through the floor even though they and the floor have mesh colliders, and the falling objects have mesh colliders. But when these objects touch another object on the way down, they stop falling through the floor. This is very strange, and I was wondering if anyone would know what to do.

Have you checked if the mesh colliders are convex?