Momento design patern with composite patern

I have a rectangle class that extend a form class, I have a composite class that extend form class, I have a momento class to do a roll back on x and y atrribut of the form. I have a canvas and I am drawing some shapes on it. by moving one shape it's ok I can undo that move by installing the momento. The problem is when I group shapes in a composit object of forms, and I move them, I can't find the way to roll back the move action by installing a momento. Does anyone know how to do that?