If some of your workstations disappeared from the list of computers available for backup, try this:
You can read how to install and configure agents for Windows in the User Manual of Handy Backup.
Related solutions: Handy Backup Server Network.
See also: FAQ | Network Server Backup:
⇒ Why workstation contents are empty after selecting a Backup Network plug-in?
⇒ My network backups are running very slowly. What can I do?