OnMouseDrag() stopped working after few clicks

I am facing a problem. After a few mouse clicks, the function stops working... I am using the OnMouseDrag() function for rotating an object in the center of my scene. The script is attached to the gameObject, the gameObject has a BoxCollider. What is the problem? Code:

public float PCRotationSpeed = 10f; public float MobileRotationSpeed = 0.4f; public Camera cam;

 void OnMouseDrag()
 {
     Debug.Log("OnMouseDrag() - START");
     float rotX = Input.GetAxis("Mouse X") * PCRotationSpeed;
     float rotY = Input.GetAxis("Mouse Y") * PCRotationSpeed;

     Vector3 right = Vector3.Cross(cam.transform.up, transform.position - cam.transform.position);
     Vector3 up = Vector3.Cross(transform.position - cam.transform.position, right);
     transform.rotation = Quaternion.AngleAxis(-rotX, up) * transform.rotation;
     transform.rotation = Quaternion.AngleAxis(rotY, right) * transform.rotation;
     Debug.Log("OnMouseDrag() - END");
 }

 void Update()
 {
     // if we are using mobile we need to detect the user touch input, if pc we do not need
     foreach (Touch touch in Input.touches)
     {
         Debug.Log("Touching at: " + touch.position);
         Ray camRay = cam.ScreenPointToRay(touch.position);
         //make sure we are touching the object and not around it.
         RaycastHit raycastHit;
         if (Physics.Raycast(camRay, out raycastHit, 1))
         {
             if (touch.phase == TouchPhase.Began)
             {
                 Debug.Log("Touch phase began at: " + touch.position);
             }
             else if (touch.phase == TouchPhase.Moved)
             {
                 Debug.Log("Touch phase Moved");
                 transform.Rotate(touch.deltaPosition.y * MobileRotationSpeed,
                     -touch.deltaPosition.x * MobileRotationSpeed, 0, Space.World);
             }
             else if (touch.phase == TouchPhase.Ended)
             {
                 Debug.Log("Touch phase Ended");
             }
         }
     }
 }