All sub categories not displaying on mobile

Site URL: https://www.skalarsystems.com/en/home

Hi there, 

I have designed a website with different category options in the menu bar. Under some of the categories, there are sub categories. 

Problem: With the desktop view, all the categories display. On mobile however, only one of the subcategories from each category displays. 

Does anyone know how to resolve this? 

The reason, this code

#header .header-menu-nav-item:nth-child(1) {
    display: none;
}
#header .header-menu-nav-item:nth-child(3) {
    display: none;
}