Rigidbody falls through floor

I know this was asked like a gazillion times before, but I’ve read many of the answers (which are obviously very similar) and I still can’t seem to figure it out.

I have a simple floor (Cube) with a box collider (isTrigger is unchecked).
I have a Sphere, with a sphere collider (isTrigger is unchecked), with a Rigidbody.
The colliders of the 2 objects don’t overlap.
The collider of each object covers the whole object.
But still the sphere falls through the floor!!!

What am I doing wrong here? :frowning:

Anyone can help?