Dear Edward,
Styled Store comes with separate menu location for desktop and mobile menu. Primary menu is meant for deskstop wheras mobile menu is only meant for mobile. For this you can go to:
Go to dashboard -> Appearance -> select you menus -> scroll down and you will get menu setting -> assign to specific menu location:
http://prntscr.com/ftq8jh
Let us know if this provides you a solution otherwise please provide us your site url with proper screenshot. So we can provide you a appropriate solution.
Thanks
