It seems that when using header menu it breaks up into 2 lines, how can they always remain on 1 line?
Header contains two sections: logo and menu items. Each one of them regarding to their nature has their own width. When your menu items can't fit in considered width because of insufficient space (high number of menu items cause this) some of menu items will go to next row.
1. Use sub-menu for menu items which have less priority
2. Decrease padding of menu by css, this one needs working with inspect element: http://webnus.biz/themes/papillon-wp/dox/#inspect