If you don’t see the WPBakery Page Builder 'Backend Editor' button on a page in the theme backend, then you will have to switch on WPBakery Page Builder for that page.
Browse to WPBakery Page Builder → Role Manager → Administrator
Go to Post Types and select Custom from the dropdown list and you will now see a list of custom post types that get used in the theme.
Here you can now switch WPBakery Page Builder on for that page as shown below.