I made a box player with a gun as a child ( fxb imported ),
the gun collision is a mesh collider, it works and detect collision with objects that have rigidbody component.
The Problem is the gun do not collide with my floor cube ( the floor object is a cube with a box collider.
Small objects can sometimes have problems colliding with larger objects.
One thing you can try is upping the iterations: Edit → Project Settings → Physics → Solver Iteration Count
Other than that, not sure since you haven’t really given that much information. Such as how are you controlling the cube, rigidbody or charactercontroller? More you give the more likely you’re to receive help. On the other hand not many people bother helping new people around here.
If I had to guess, the cube wouldn’t be moving because it’s transforms are being controlled by the parent object and the floor isn’t moving because it’s not a rigidbody, it would collide with the spheres because the spheres are rigidbodys.