Whatever it is, it is probably related to the content you placed in some pages.
the theme is fully responsive, but nothing stops you from adding content in some areas that is not and will break responsiveness. For example if you use a fixed width on an object, it would break responsiveness. you should use max-width or width in %…
I don’t see anything on your home page, so may be google gives you an indication about which page has content wider than screen. If not, you need to inspect every page, shrink your browser and see if one has content that ends up cutoff when you do so.
Once you found the page with the problematic content we can help figure out how to make it responsive