Twenty Fifteen Header to top across page

Hi, I am trying to customize Twenty Fifteen with a child theme. I was able to move the left sidebar to right and the header to top; however, I cannot make the header stretch across the page; the sidebar on the right seems to occupy the space from top to bottom stopping the header to move across to right. When I view the .site-header on the browser (with Inspect Element), I can see it moving across to right (in principle) but something seems to be blocking its way and I cannot figure out what. Any ideas what might be the issue? Thanks for your help.