Java SE (Java Platform, Standard Edition)

I have a JPopupMenu that displays some info for me and I would like to add a submenu but it doesn't seem to work. I can successfully add the submenu but it doesn't "expand" when I mouse over the submen.

JPopupMenu pm = new JPopupMenu();
...
JMenu submenu = new JMenu("submenu");
JMenuItem jmi = new JMenuItem(...);
submenu.add(jmi);
...
pm.add(submenu);

The popup menu looks as it should and the only thing that is missing is the action that should expand the submenu when I mouse over (and clicking doesn't work either).

Any ideas?