Im having trouble with OnTriggerEnter and OnTriggerExit

I have it set so that my player becomes a child of a platform when the player enters the trigger (which is an empty game object above the platform). Once this happens though I am unable to exit the trigger and remains parented to the platform. The point of the code is for the player to be parented to a platform so he/she does not fall off the platform while it moves and to become un-parented from it when they leave the trigger object. The following is my code:

var player : GameObject;
var platform : GameObject;
function OnTriggerEnter(trigger : Collider)
    {
       print("enter");
       player.transform.parent = platform.transform;

    }


function OnTriggerExit(trigger : Collider)
    {
       print("exit");
       player.transform.parent = null;
    }