This could be happening because of one of the following reasons:

• The object you are trying to manipulate is assigned to tasks, so you will have to use Edit as Assigned> rather than Edit> and define its positions as assigned to different tasks.
  You can also choose to un-assign, manipulate, then re-assign.
• If the object is assigned to a 3D path, then its position will be dictated by the 3D path and you can't manipulate the object using the manipulator.