Begginer Jumping javascrpit problem.

I am making my first game in unity and do not have a lot of coding experience with java script. The game i can making is a simple ball rolling platformer. If i spam the jump button (w) my ball jumps the first time then when it lands it does not jump right away but it jumps randomly after a number of button presses. Here is the code.

pragma strict

 var rotationSpeed = 100;
 var jumpHeight = 8;
  
 private var isFalling = false;
  
 function Update ()
 {
     //Handle ball rotation.
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
     rotation *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.back * rotation);
    
     if (Input.GetKeyDown(KeyCode.W) && isFalling == false)
     {
         rigidbody.velocity.y = jumpHeight;
     }
     isFalling = true;
 }
  
 function OnCollisionStay ()
 {
     isFalling = false;
 }