We have a site with multiple sub-sites. For each sub-site, there is a secure area where administrators or general logged in users can access to view additional information. The administrators can currently login correctly to their respective subsites but for some of them, whenever they try to manage content on the site e.g. add new article, they are presented with another login prompt.
It appears that this behaviour is not consistent for all users as some do not experience this issue even if the same browser (Chrome) is used.
Does anyone have any ideas what can be causing this and how we can fix it?

Thanks.