weird mouse drag behavior

I wish to make a checker game. Now I am trying to implement draggable game pieces. Here are the scripts attached to my piece GameObject:

void OnMouseDrag(){
	Debug.Log(gameObject.transform.position + " " 
		+ Camera.main.ScreenToWorldPoint(Input.mousePosition) + " " 
		+ Input.mousePosition + "

Vector3 convertedPos = new Vector3(Camera.main.ScreenToWorldPoint(Input.mousePosition).x,
gameObject.transform.position = convertedPos;

I have an orthographic looking-downwards camera at (0,70,0).
When I tried to drag the pieces, in some cases it works fine. In other case the pieces moved are not the ones I dragged.

So i assume you have the right z parameter which is distance between camera and the object.
If not try this:

private float floatZ;
void OnMouseDown()
        floatZ = Vector3.Distance(Camera.main.transform.position, this.gameObject.transform.position);

    void OnMouseDrag()
        this.gameObject.transform.position = Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, floatZ));

Just for reminder OnMouse doesn’t work on mobile, use Input.GetMouseButton instead , this works on multiple platform.But it can not automatically select your object

i fixed it.
the capsule collider of some pieces are not correctly sized.