Problem with the randomly spawning enemies

Only the latest zombie is responsive to player movement and shooting. When I kill a zombie, the last zombie is dying. Also when I approach another zombie(not the last one), it is not attacking. I think the problem is not in the script.

The zombie spawning script ;

 private int delay=2;
 public GameObject zombie;
 private Vector3 newPosition;

 void Update()
 {       
     if(delay<Time.timeSinceLevelLoad)
     {
         newPosition = new Vector3(-15.03345f, 2.287f, Random.Range(-5f, 8f));
         Instantiate(zombie, newPosition,
             Quaternion.Euler(zombie.transform.rotation.x, zombie.transform.rotation.y, zombie.transform.rotation.z));

         delay+=5;
     }       
 }