Mobile issues with child theme – menu missing etc.

Hi folks. I’m using the latest WordPress and a child theme for Responsive. I just noticed some major mobile display issues; on mobile devices the dropdown menu disappears entirely and there are some odd scaling / stacking issues with the widgets.

I used “live preview” with the parent theme and the scaling and menu issues didn’t appear, so I know this is something with my child theme.

I’ve only made a few edits, including forcing my own header image into the header, and changing some colours etc. in the style sheet. I can’t see anything I would have done that would cause the layout to break so spectacularly, unless in my child’s style.css I’m failing to call something correctly. Can anyone make a suggestion please?