Adding system menu items: doesn't work when the form is maximized

I am adding menu items to a MDI child form's system menu (Delphi 5) using:

      SysMenu := GetSystemMenu(Handle, False);
      AppendMenu(SysMenu, MF_SEPARATOR, 0, '');
      AppendMenu(SysMenu, MF_STRING, WM_USER + 1, 'Item 1...');

etc. and trapping WM_SYSCOMMAND.

This works fine, but when the form is maximized, WM_SYSCOMMAND does not even
get called when the custom items are clicked.  It does get called when the
form is put back to normal status.

Any ideas???