Thanks Andrew, appreciate the response.
I've changed Pin for Target, set relative to browser, and have the same issue in that the stack is behind the navbar, where I'd like it in front, I've published to the same link as before. Z index on target is set higher than that of the navbar.
Interestingly when I change the setting to be relative to body, the stack does show above the navbar, but obviously disappears when the page scrolls.
Ideally I want to have the button starting on, or just above the navbar, always visible and in view when the page scrolls. I guess my work around is to have the starting position just below the navbar, but I'm keen to understand how to resolve.