I'm not sure what the problem is, sorry i can't run tests right now.
But if you are trying to make your ball jump, i would use AddForce()
http://docs.unity3d.com/ScriptReference/Rigidbody.AddForce.html
And instead of checking if it's jumping, i would check if it is grounded and only execute the Jump(AddForce) Code block when it IS grounded so it will not constantly addForce().
The instructions would be:
//IF JUMP KEY IS PRESSED
// ADD FORCE AND SET GROUNDED TO FALSE
//IF THE BALL TOUCHES THE GROUND 'OnTriggerEnter'
//SET GROUNDED TO TRUE
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