I changed the ground to using a mesh imposter. It’s working, so there must be an error in your code:
https://playground.babylonjs.com/#NKVJ46#1
EDIT: Even if I give the sphere a meshImposter too, it’s working.
EDIT:
Game.Ball.physicsImpostor = new BABYLON.PhysicsImpostor(Game.Ball, BABYLON.PhysicsImpostor.SphereImpostor, {mass: 0});
Your sphere imposter needs a mass.