After a host or virtual machine failure, a virtual machine might not be restarted.
When a host fails or a virtual machine fails while its host continues running, the virtual machine might not restart or restarts only after a long delay.
vSphere HA might not restart a virtual machine after a failure or might delay its restart for several reasons.
Virtual machine is not protected by vSphere HA at the time the failure occurred
Insufficient spare capacity on hosts with which the virtual machine is compatible
vSphere HA attempted to restart the virtual machine but encountered a fatal error each time it tried.
Your cluster's shared storage is vSAN and one of the virtual machine's files has become inaccessible due to the occurrence of more than the specified number of host failures.
Restart actually succeeded.
To avoid virtual machine restart failures, check that virtual machines become protected by vSphere HA after they are powered on. Also, ensure that your admission control settings match your restart expectations if a failure occurs. Maximizing the compatibility between virtual machines and hosts in the cluster can also reduce the likelihood of restart failures.
For information on the factors vSphere HA considers for virtual machine restarts, see "Determining Responses to Host Issues" in vSphere Availability.