I have had that happen in the past, not specifically with Sierra though. The only way I could fix it was to reformat and start fresh. I never discovered why it wasn't working. Sidebar: I really didn't need the hourly backups and other cron controllers were deemed too unreliable, so I decided to stick with Carbon Copy Cloner using the Safety Net, which caches modified and deleted files.
I backup to a Mac Mini server running the Server OS on my home network and the first few backups after Sierra were slow and painful. It seemed like they were forever looking for the backup disk. I just let it do it's thing and now it's running as expected.