I think what you need, if i did not missundertood, is to set the bounce property to non-zero value (this property can be set from inpector). if you want the ball to bounce in a perfectly "elastic" way, and never stop bouncing, put bounce in 1. If you want the bounce to be more "plastic" and the sucesive bounces to be decreased in magnitude, so put bounce to a lower value, like 0.3.
Does this help?