Mobile site not displaying header

I’ve designed a site with two headers - one for PC’s and Tablets, the other for smartphones. In the Blocs preview everything is fine. The Mobile header only shows when switching to mobile view and is invisible when tablet and PC view is selected. The problem is, when exporting the site and viewing in a local browser, adjusting the browser width to see the mobile version doesn’t display the mobile header - its just not there at all. No background image and no content - its almost as if the bloc wasn’t there at all.