Mobile-menu / Main-menu

When I drag my browser-window narrower to simulate tablet-screens :
– narrowing up to +-60% of width everything works OK …
– but narrow more, and the Main-menu disappears altogether …
– (not talking about the “top-menu” … but the Main-menu specifically)
– until it is only +-35% wide at which point the Mobile-menu pops in at the top

So between 35% and 60% my site shows no menu …

Here’s the question :
– Is there any code/css to alter to make the “Mobile-menu” activate/pop-in earlier ? (effectively then showing for tablets as well as mobile phones)