Rolling ball gets stuck in corners

I'm moving a ball on a flat surface, limited by stretched cubes, by changing the direction of gravity. Works nicely, except, when the ball ends up in a corner, it's stuck. It seem that once the ball reaches velocity = 0, it's stuck forever. Here's the gravity code:

function FixedUpdate() { xAngle = Input.acceleration.x; yAngle = Input.acceleration.y;

Physics.gravity = Vector3(-yAngle*10,-10,xAngle*10);

}

Got it. The ball falls asleep and can be kept awake by adding a force, even if the force is zero Rigidbody.AddForce.