I did some more tests. Below are my findings:
1) If a level N menu item is added by AngularJS, then its sub-level (level N+1) menu will not show up. (N = 1, 2, ..).
2) It seems that even the initialization function “$(‘#main-menu’).smartmenus(); ” (not refresh) cannot handle the menu item added by AngularJS if the item contains sub-menu.
3) The pure SmartMenus (without Bootstrap) has the same problem.