Collider2D of game object does not work, unless I duplicate the game object.

I am trying to implement one way platforms in my "game", I am doing it like this:

 public class OneWayPlatform : MonoBehaviour {

 public Transform player;
 private Collider2D collider;
 private float halfTheSpriteSize;
 private float halfThePlayerSize;
 public bool isPlayerAbove;

 void Start()
 {
     collider = GetComponent<Collider2D>();
     halfTheSpriteSize = collider.gameObject.GetComponent<SpriteRenderer>().bounds.size.y / 2;
     halfThePlayerSize = player.gameObject.GetComponent<Collider2D>().bounds.size.y / 2;

     if (player == null)
     {
         player = GameObject.FindGameObjectWithTag("Player").transform;
     }

 }
 // Update is called once per frame
 void FixedUpdate () {
         Debug.DrawLine(new Vector3(player.position.x, player.position.y - halfThePlayerSize, 0), 
             new Vector3(collider.gameObject.transform.position.x, collider.gameObject.transform.position.y + halfTheSpriteSize, 0), Color.red);
     this.isPlayerAbove = ((collider.gameObject.transform.position.y + halfTheSpriteSize) - (player.position.y - halfThePlayerSize)) < 0;
     if (isPlayerAbove)
     {
         collider.enabled = true;
     }
     else
     {
         collider.enabled = false;
     }
 }

 }

This solution does not work... UNLESS I copy the platform and move it over, then it works perfectly, sometimes...

Is this way of thinking good at all? Is calculating distance on y axis a good solution to make one way platforms?