Are you trying to change the position of the field group after you have edited a post where the fields are displayed?
If this is the case. WP stores the locations of all meta boxes when you edit a post, even if you have not moved them. This order is stored in the _usermeta table with the meta key “meta-box-order_{$screen}”.
The only way to move the boxes after you have edited an object containing those boxes it to move them manually or to delete this entry in the DB. There is a plugin that can be used, but it only works on posts and pages. https://wordpress.org/plugins/reset-meta-box-positions/
