They could be failing, leaving a folder in place like /backups/<website-id>/wip-snapshot* for every time it attempts to make the backup. If everything is working fine, there should be zero wip* folders. If there are some, then try the following in de Control Panel:
- under Settings/Service, change the minimum and maximum age to something else, and after that change it back to what it was to ensure each server gets told (again) what the intervals are
- increase the Maximum Concurrent Backups
- do a manual backup for that website to get the train moving
If it doesn't start behaving normally again after that, drop a ticket to the support team, there's more people that had this issue and they were looking to reproduce it reliably for a fix.
