Second level navigation issues

I have quite a long second level navigation which goes down past the screen. When at the top of the website and I hover to show the second level nav it shows but when I scroll it disappears. When I hover again, after scrolling a little, the second level menu appears again but it doesn’t scroll with the website so I can’t see further than 6 menu items.

I am replicating this website from the Avada theme to Vantage. In Avada, while hovering on the second level menu, the second level menu scrolls with the website so all the menu items can be seen. I hope I have explained this properly and someone can help/advise.