Groups / Elements are not fixing in position in a responsive app

Hi there, this is probably super simple so hoping for some help.

I can make the app responsive (full width) fine, but when I add elements or groups etc. on the page - when I open up the preview - they are now positioned strangely in the preview, and not fixed as per how I positioned them on the page when building that page.

Is there a way to make them relative to another element, such as the header or sidebar, or another way to ensure they are fixed in place, as per the design?