Enable dropdowns for all the menus, not main menu only?
Possible Answers to Your Question:
The top menu is limited to one level for user experience reasons – if the header contains a main menu or other content, drop-downs in the top menu would cover them. It would also add option and design complexity to responsive modes where the top menu is not shown at all. You would need to implement this in your child theme by modifying the wp_nav_menu arguments in the corresponding templates, ie in header-secondary.php for the Top menu you have:
wp_nav_menu( array( 'theme_location' => LAYERS_THEME_SLUG . '-secondary-right' , 'container' => 'nav', 'container_class' => 'pull-right' , 'menu_class' => 'nav nav-horizontal', 'fallback_cb' => FALSE ) );
Where you can add
'depth' => 1,