[wp-db-backup] Scheduled backups stopped working

I have this plugin installed on MU and am having 5 sites backed up daily. The backups are being emailed to me, but one-by-one they are no longer sending. About 2-3 months ago, 2 of the backups just stopped being emailed. I’ve slacked on finding the solution (an subsequently, backing up the sites). Then 2 days ago, another site stopped backing up as well. All sites were created at the same time and none have been changed or updated since they went live in August. Any idea why these backups have stopped?