Ignore collision on tag not working

HELP...

When my player hits a collider - a set of game objects are instantiated. Before he reaches it - an NPC walks through this collider, and I don't want him to trigger these instantiations.

I am using this script to ignore the collisions (below): the first time I used it, it worked and the scene wasn't instantiated. Now, it isn't working any more, and I have not made any changes to this part of the game! This collider has been acting strangely for the past few days, sometimes only instantiating some objects when the NPC walks through, and sometimes all of them. I really can't understand what's going on!

 var npc : GameObject;
 
 function Start () { 
 
  var ball = GameObject.FindGameObjectWithTag("Ball");
 
   Physics.IgnoreCollision(npc.collider, collider);
    
    Physics.IgnoreCollision(ball.collider, collider); 
 
        }

I originally used OnTriggerEnter - which worked a few days ago, but now doesn't at all (and looking around online it seemed Start was a better idea. This is the script I'm using to instantiate the objects, in case that helps (the commented out part was where I tried to get the collider to ignore the NPC collision within this script, but that doesn't work.

 var plane : GameObject;
 var bird : GameObject;
 var flockControllerGameObject : FlockController;
 var planeSounds : AudioClip;
 //var npc : GameObject;
 
 
 function OnTriggerEnter (other : Collider) { 
  
   //var ball = GameObject.FindGameObjectWithTag("Ball");
 
   //Physics.IgnoreCollision(npc.collider, collider); 
  // Physics.IgnoreCollision(ball.collider, collider); 
 
  if(other.gameObject.tag == "Carl")
 
 
     bird.SetActive (true);
     flockControllerGameObject.ReviveBirds();
     plane.SetActive (true);
     plane.transform.position = Vector3(590.788,125.2536,224.8894);
     planeSounds = plane.audio.clip;
     audio.clip = null;
     plane.audio.clip = planeSounds;
     plane.audio.Play();

Does anyone know whether its my computer that's playing up - or is there something wrong with the script? Thanks in advance, Laurien