Not able to show sub-menus (grand-child) using {{page.primary_menu}}

I am creating a drupal 8 custom theme. I am not using any base theme. However, in the main menu, sub menu items which link to different pages of website, are not coming in browser. If I remove the link from sub menu to root and make it direct child of main menu , then the link gets displayed. How can I work with menus(also sub menus) and add classes to them.