Detect Object At on Vector2

I am spawning instantiated objects which stacked on top of each other. I want to locate the stacked object by click. I am using overlapcircleall bu t it seems like it is not working.
Hope you guys can help.

My code snippet:

private Vector2 fp; // first position
    private Vector2 lp; // last position
    void Update () {
    		if (Input.GetMouseButtonDown (0)) {
    			fp = Input.mousePosition;
    			Debug.Log ("Button pressed at "+ fp );
    			Collider2D[] hitColliders = Physics2D.OverlapCircleAll(fp, 0, Physics2D.DefaultRaycastLayers,Mathf.Infinity,Mathf.Infinity);
    				foreach (Collider2D collider in hitColliders) {
    				Debug.Log ("Collided with: " + collider);
    			/*if (Input.GetMouseButtonUp (0)) {
    				lp = Input.mousePosition;
    				Debug.Log ("Button released at "+ lp);

As it is stated in Input.mousePosition description, it returns you mouse coordinates on the screen in pixels, not the world coordinates in units which you need for Physics2D.OverlapCircleAll.

What you need to do is to convert those screen coordinates in world space. You can use Camera.ScreenToWorldPoint method for that purpose. Just google it, there are a lot of examples around, I don’t think it’s necessary to explain much.