Animation problem

My characters whole arm wont animate at all, if I attach a gameobject to it at runtime.

 var bone = GameObject.FindGameObjectWithTag("Hand");
 CurrentWeapon = GameObject.Instantiate(CurItem.Gameobject,Vector3.zero,Quaternion.identity);
 CurrentWeapon.transform.parent = bone.transform;

 CurrentWeapon.transform.localPosition = Vector3.zero;
 CurrentWeapon.transform.localRotation = Quaternion.identity;
 CurrentWeapon.transform.localScale = Vector3.one;