Something has not started

An update round, a backup, a restore or a health check you started has not begun.

Every operation on a site runs one at a time, and some of them wait for the site itself. A delay has one of three causes:

Another operation on the same site. A backup started while an update round runs waits for the round. The site's Health page lists the work waiting for it.
The site is unreachable from the platform's side. Work the platform cannot push is handed to the site over the site's own calls, which it makes every fifteen minutes on its WP-Cron — so a backup on a firewalled site begins on the site's next call, not at once. Connection problems.
The platform is busy. Operations start in order; a brief pause on the platform's side delays a start by a minute or two. An operation the platform's own work was interrupted on ends as such and is run again.

An operation that has not begun after an hour with none of these applying is worth reporting, with the site and the time.