OnDrop get called if I drag the object quickly but not if I drag it slowly !!!

My drop zone code is as simple as :

Code (CSharp):

1. public void OnDrop(PointerEventData eventData)
2. {
3. Debug.Log("item was dropped");
4. }

The object that's being dragged has the following code :

Code (CSharp):

1.     public void OnDrag(PointerEventData eventData)
2.     {
3.         var mouseScreenPoint = new Vector3(eventData.position.x, eventData.position.y, transform.position.z);
4.         mouseScreenPoint = Camera.main.ScreenToWorldPoint(mouseScreenPoint);
5.         transform.position = new Vector3(mouseScreenPoint.x, mouseScreenPoint.y, transform.position.z);
6.     }

and it has a Canvas-Group-Component attached with Block raycasts set to false,

If I drag the object and drop it quickly the code works ! if I drag it a little more before I drop it, it doesn't work !