Why doesn't the OnTriggerStay2D run?

Why doesn't the OnTriggerStay2D run?? The object that collides with the trigger has both a collider and a rigidbody attached. (does it have to have a rigid body?)

 #pragma strict
 public var hasGravity : boolean;
 public var mass : float = 5;
 public var reachFactor : float = 5;
 
 function Start () {
 gameObject.AddComponent(CircleCollider2D);
 gameObject.GetComponent(CircleCollider2D).isTrigger = true;
 gameObject.GetComponent(CircleCollider2D).radius = mass * reachFactor;
 }
 
 public function OnTriggerStay(coll: Collider) {
     Debug.Log("Sent");
     if (coll.gameObject.GetComponent(affected_by_gravity)) {
         coll.gameObject.GetComponent(affected_by_gravity).pull(mass, Vector2(gameObject.transform.position.x,gameObject.transform.position.y));
     }
 }