Prefab Instantiate

I have written simple code to instantiate prefab at runtime.

I have array of prefab. Using Random.Range , generating random number to instantiate prefab. But prefab does not change though different random number is generated.

Code:

 Public var bubblePrefabs : GameObject[];
 private var futureBubble : GameObject;
 
 function Start () {
     futureBubble = GameObject.Find("Sphere(FutureBubble)"); // Next bubble to be shoot
     if(futureBubble != null)
     {        
         CreateRandomPrefab(futureBubble.transform.position);
     }
 }
 
 function Update()
 {
     var spawnPoint : GameObject = GameObject.Find("Bullet");         // Current Bullet Bubble to be shoot
         
     if(Input.GetMouseButtonDown(0))     
     {     
          CreateRandomPrefab(futureBubble.transform.position);
          if(spawnPoint != null)
          {
              Instantiate(bulletSphere, spawnPoint.transform.position,spawnPoint.transform.rotation);
          }            
     }
 
 }
 
 function CreateRandomPrefab(position : Vector3)
 {
     random = Random.Range(0 , bubblePrefabs.Length);
     Debug.Log("Random :" + random);
     var bubble : GameObject = Instantiate(bubblePrefabs[random]);
     bubble.transform.position = position;
 }

But it change color only two times . I want to instantiate every time i click mouse button. So what is the problem? Please help me..

Thanks for ur support and help in advance..