Do you see this?
The “Advice, Design, etc” are their own page. “Sectors” is it’s own page, but Retail and others, are inside of it. But Sectors is not a direct link. The nav menu is fixed to ‘top of the browser’, so when the link for retail, etc is clicked the link is going to the right place. But the browser is making an adjustment for you after seeing the menu is over it! Does that make sense?
If you hide the nav menu you would see it correctly. Pretty sure, even if you add bottom margin to the nav menu, it still has a higher z-index of 1000, so it’s always going to show over that section.
I would make “Section” menu in nav a link to itself. And remove the drop-down from that link. It’s not needed, the user can just scroll down, just like they do on Contact page.
Keep the drop-down and change the link setup to this screenshot. Then each link will ‘Scroll’ to the section: