Problem with Prefab - Following cursor

Hi all , i want to my cube prefab will instantiate on my cursor position when i make a first left click and after that i want to the cube follow my cursor , after , on a second left click i want my cube get a gravity scale equal to 5 but it didn't and i don't know why ^^ ... thx for your help ! :-)

here is my 2 scripts :

        public class DropObject : MonoBehaviour {

 public Vector3 screenPoint;
 public Vector3 offset;
 Vector3 pos;
 public GameObject Cursor;

 public GameObject PrefabCaisse;
 public int IsDrag;

 private float fireSpellStart = 0f;
 private float fireSpellCooldown = 2f;

 void Start () {
     IsDrag = 0;
 }

 void Update () {

     Vector3 curScreenPoint = new Vector3 (Input.mousePosition.x, Input.mousePosition.y, 0);
     Vector3 curPosition = Camera.main.ScreenToWorldPoint (curScreenPoint) + offset;
     Cursor.transform.position = (Vector2)curPosition;
 
     if (Input.GetButtonDown ("Fire1") && Time.time > fireSpellStart + fireSpellCooldown && IsDrag == 0) {
         Instantiate (PrefabCaisse, Cursor.transform.position, Quaternion.identity);
         fireSpellStart = Time.time;
         IsDrag = 1;
     
     } else if (Input.GetButtonDown ("Fire1") && IsDrag == 1) {
         IsDrag = 2;
     } 
         } 



            public class DragAndDropMover : MonoBehaviour {

 public Vector3 screenPoint;
 public Vector3 offset;
 Vector3 pos;

 bool Ignore;
 // bool IsOver;
 public int IsDrags;

 public GameObject PrefabCaisse;
 void Start () 
    {
     IsDrags = 1;
 }
 
 // Update is called once per frame
 void Update () {
 
     GameObject DropObj = GameObject.Find ("GameManager");
     DropObject DropObjScript = DropObj.GetComponent<DropObject> ();
      IsDrags = DropObjScript.IsDrag;

     Debug.Log (IsDrags);

     if(IsDrags == 1) {
         Dragged();
     }
     if (IsDrags == 2) {
         PrefabCaisse.GetComponent<Rigidbody2D> ().gravityScale = 5; 
     }
   void Dragged()
 {
     Vector3 curScreenPoint = new Vector3 (Input.mousePosition.x, Input.mousePosition.y,0);
     Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
     PrefabCaisse.gameObject.transform.position = (Vector3) curPosition;
 
     GameObject player = GameObject.Find ("Player");
     Player PlayerScript = player.GetComponent<Player> ();

     Physics2D.IgnoreCollision (PrefabCaisse.GetComponent<Collider2D> (),player.GetComponent<PolygonCollider2D>(),Ignore = true);
     Physics2D.IgnoreCollision (PrefabCaisse.GetComponent<Collider2D> (),player.GetComponent<BoxCollider2D>(),Ignore = true);
 }
    }