Hi,
It is possible that your WordPress scheduler is broken. Please could you install the free WP-Crontrol plugin, then go to Tools->Cron Events? Are there any error warnings on that page?
Could you also send us a copy of the automatic backup log? This can be found in the UpdraftPlus Existing Backups tab. The contents will be too long to post here directly, but you can use an online service, such as Dropbox or Pastebin, and post the link here. You can also set your reply to ‘private’ if you prefer.
Best Wishes,
David N