Make sure the Hangouts Chat for Confluence user has access to the pages that are updated. For example, if you want to send notifications for a Space that has restricted access, make sure to add the “Hangouts Chat for Confluence” user to the list of users that have “View” permissions:

Another issue can be the permissions set on an individual blogpost, page (or parent page), where you would need to grant the “Hangouts Chat for Confluence” user “View” permissions as well: