CirrusMost HA configurations fail because of communication issues, typically in the misconfigured HA communication settings realm. Common causes include IP address conflicts, HA VLAN misconfiguration, mismatched software versions, and devices/systems not reachable on the network. Have you tried all the troubleshooting tips described in K13946: Troubleshooting ConfigSync and device service clustering issues? If not, I would start there. Also, there's a pretty good video on adding a new device to an existing device group on the F5 YouTube channel here: https://www.youtube.com/watch?v=Auc13Q31qUA Although this is for a Sync-Only device group, the steps are similar.
