If the cluster resource pool tree is not internally consistent (for example, the sum of the children's reservations is greater than the parent pool's nonexpandable reservation), the cluster does not have enough resources to satisfy the reservations of all running virtual machines making the cluster red. This can occur if vCenter Server is unavailable or if resource pool settings are changed while a virtual machine is failing over. To address this problem, revert the associated changes or otherwise revise the resource pool settings.

If a cluster enabled for HA loses so many resources that it can no longer fulfill its failover requirements, a message appears and the cluster's status changes to red. The cluster attempts to failover virtual machines in case of host failure, but is not guaranteed to have enough resources available to failover all virtual machines covered by the failover requirements.

Examine the list of configuration issues in the yellow box at the top of the cluster Summary page.