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?

I fixed the issue. The key was here:

this.isPlayerAbove = ((collider.gameObject.transform.position.y + halfTheSpriteSize) - (player.position.y - halfThePlayerSize)) < 0;

Due to incosistent sizes of sprites and colliders in my scene (this is a prototype), even though I thought it should be zero, it was not. To correct this issue I used:

this.isPlayerAbove = ((collider.gameObject.transform.position.y + halfTheSpriteSize) - (player.position.y - halfThePlayerSize)) <= 0.1f;

The solution might not be optimal, but Im staying with the KISS rule for my prototype - and learning example.