Language flags disappear on single post

I have a 2 languages site and I added polylang language switcher in the custom menu. However, flags disappear on the single post site. My main menu is formed by pages and categories (as sub items). When you click on the single category and it shows post summary – language switcher works. But when you click on this post to open the whole content, flags are not anymore visible ?? How could I fix it?

Thanks and regards, Jagoda