OK. I think I might have just found the cause of some of my issues by accident. My site is running entirely over SSL (using Lets Encrypt) and I just spotted some Mixed content warnings both on Page Builder load, and on actually loading the site.
Mixed Content: The page at ‘https://www.domain.com/wp-admin/post.php?post=1686&action=edit’ was loaded over HTTPS, but requested an insecure stylesheet ‘http://fonts.googleapis.com/css?family=Raleway:400’. This request has been blocked; the content must be served over HTTPS.
Is this something that can be fixed?