GameObjects escaping somewhere and losing its collider when touch and drag in iOS

Hi all,

Am trying to touch and drag my game object to some places in screen. Some times the game object is escaping some where that means out of bounding boxes and its losing its collider. i dont know why its happening ? if any one face this same problem previously kindly provide me suggestions. and here is my code for touch and drag....

Code :

 Touch touch = Input.GetTouch(0); Ray ray = Camera.main.ScreenPointToRay(Input.GetTouch(0).position); RaycastHit hit; if (collider.Raycast (ray, out hit, 100))  {     if(hit.collider.gameObject.name == "box")     {
         if (touch.phase == TouchPhase.Began)          {    
             screenPoint = Camera.main.WorldToScreenPoint(transform.position);
             offset = transform.position - Camera.main.ScreenToWorldPoint(new Vector3(touch.position.x, touch.position.y, 0));   
         }         else if(touch.phase == TouchPhase.Moved)         {
             Vector3 currentScreenPos = new Vector3(touch.position.x, touch.position.y, 0);
             Vector3 currentPos = Camera.main.ScreenToWorldPoint(currentScreenPos)
 + offset;
             transform.position = currentPos;         }       } }

Thanks in advance.