If I understand what you are describing, this is the intended behavior of a physics object. Jumping higher would be like creating energy from thin air :)
My suggestion is: don't use a bouncy material! in fact, use a material that completes eliminates bouncing. Then you add force at each impact directly from script. Each time the ball touches the ground, you apply an F amount of force upwards. I think this would get you the effect you want.
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