What is a possible cause if a vSphere administrator sees the alarm: vSphere HA virtual machine failed to failover?

A possible cause for the alarm "vSphere HA virtual machine failed to failover" is indeed that the host is still running but has disconnected from the network. In a vSphere High Availability (HA) environment, if a host becomes isolated and unable to communicate with the vCenter Server or the rest of the cluster, the HA mechanism may trigger this alarm since it cannot execute the failover process necessary to keep the VMs operational on another host.

In this scenario, while the host is operational and the virtual machines are running, their isolation from the network prevents HA from detecting a failure that would trigger the VM failover, thus leading to the alarm being raised. This situation can occur due to network issues, such as a switch failure or network misconfiguration, which prevents the host from communicating with the necessary HA services.

Other scenarios, such as incomplete cloning operations, reboots of the vCenter Server, or errors during migration, pertain to different aspects of VM management and do not directly influence the operational status of HA in the way described by this alarm.