POPUPPPP

Hi I am trying to create popups at a nesting depth and using _root at this
point does not work for me....
My code line is this

details_window = mx.managers.PopUpManager.createPopUp(_root,mx.containers.Window, false,{closeButton: true, title:"The Popup Window Title"});

When i try this same function in a flat movieStructure ,It works ...
Can anyone help me out