The sections you see will change based on the page you’re viewing. Some pages will be limited on what sections you can add. Again, this is determined by how the theme files have been coded.
To change the page you want to edit in your theme, click the down arrow next to the current page at the top of the window. Select the next page you want to edit by clicking on it. Be sure to save any changes to your current page before moving on to the next.