Course backups not running but queued

My automated course backups use to run regularly, but now I have discovered that on the page Site administration → Reports → Backups the backups are marked as either 'Queued' or 'Skipped'. The backups are stored in a backup folder on the server. There some backups actually have been running, but very often not at the scheduled time, and far from all courses are backed up.

Any ideas?

I currently use Moodle 3.11.4+