Menu doesn't work on mobile

Hi ! Just got my blog switched over to Corporate Fotografie (free) - it's www.thewaryworker.com. Very nice theme, by the way. But...the main navigation menu does not expand on reduced screen sizes (mobile as well as when I shrink down my desktop version). I deactivated all my plugins, one by one, but there doesn't seem to be any conflict. Any idea how I can fix this ? Thank you ! Kat