Are you using the latest version?
I’ve built a lot of sites with the same Theme you’re using and haven’t run into that issue. I think it’s because they added the Responsive Menu Icon in version 1.4.1 and you’re using an older version which probably doesn’t show it.
If you’ve made customizations to your CSS and your Site, then you can’t upgrade to the latest version since you made modifications to your site which would override all the work that you did…….you’ll need to adjust each part of the theme you adjusted to get everything working….(shouldn’t take that long).
The latest version shows the Responsive Icon when being viewed on a Mobile Device…..Example – http://demo.appfinite.net/epik/ Resize or View that link on a mobile device to see that it works perfectly on mobile phones.
Just let me or anyone else know what’s missing.