Hi Nathan,
Thank you for reaching out to us. The Scroll effect works when the Element comes into view or the Element leaves the viewport, since you’re using Image element in the Header so this event fires only once when the page is loaded because the Image element comes into view once and never leaves the viewport.
So basically it works when you scroll into a view and then while scrolling the element leaves the viewport. You can learn more about the Scroll effect here https://theme.co/docs/effects-module#scroll
Hope this helps!