The player ball must jump immediately -

Format your code.

I just tried your code and worked perfectly - So I'm asking you: did you attached that script to the ball or what? I'm asking because I can't find a reason to assign the ball object to a variable jumpObject if the script it is supposed to be attached to the ball. Anyhow this is your code cleaned up a little bit:

 #pragma strict
 var jumpDelay : boolean; 
 var jumpHeight = 10.0; 
 var doublejumpHeight = 7.5; 
 private var canJump : boolean; 
 private var groundTag : String = "Ground"; 
 private var doubleJump : boolean; 
 private var jumpObject : Rigidbody; 
 
 function Start()
 {
     jumpObject=this.rigidbody;
 }
 
 function OnCollisionEnter( col : Collision ) { 
     if( col.gameObject.tag == groundTag ) { 
         doubleJump = false; 
         canJump = true; 
     } 
 } 
 function OnCollisionExit( col : Collision ) { 
     if( col.gameObject.tag == groundTag ) { 
         canJump = false; 
     } 
 } 
 function Update( ) { 
     if( canJump == true && Input.Get$$anonymous$$eyDown( $$anonymous$$eyCode.Space ) ) { 
         jumpObject.velocity.y = jumpHeight; 
         doubleJump = true; 
     } 
     if( canJump == false && doubleJump == true && Input.Get$$anonymous$$eyDown( $$anonymous$$eyCode.Space ) ) { 
         jumpObject.velocity.y = doublejumpHeight; 
         doubleJump = false; 
     } 
 }