How can I enable the primary menu (or header) to be sticky? Have tried doing this with CSS, it will work for a minute and then defaults to scrolling again. Theme I am using is Monument Valley. The website url is https://twinstarhome.com/duraflametest/
I can see the sticky header working perfectly fine on your website, just like it does on theme demo website.
The sticky header only appears when you try to scroll up (when the page is already scrolled down). While scrolling down there is no real need for the header to be displayed and so the theme leaves all screen space for your website content. Once a visitor tries to scroll up again, the header appears. That’s how sticky header logic works in my themes, and there is nothing wrong with your website.