A cluster has a load imbalance of resources.
A cluster might become unbalanced because of uneven resource demands from virtual machines and unequal capacities of hosts.
The following are possible reasons why the cluster has a load imbalance:
The migration threshold is too high.
A higher threshold makes the cluster a more likely candidate for load imbalance.
VM/VM or VM/Host DRS rules prevent virtual machines from being moved.
DRS is disabled for one or more virtual machines.
A device is mounted to one or more virtual machines preventing DRS from moving the virtual machine in order to balance the load.
Virtual machines are not compatible with the hosts to which DRS would move them. That is, at least one of the hosts in the cluster is incompatible for the virtual machines that would be migrated. For example, if host A's CPU is not vMotion-compatible with host B's CPU, then host A becomes incompatible for powered-on virtual machines running on host B.
It would be more detrimental for the virtual machine's performance to move it than for it to run where it is currently located. This may occur when loads are unstable or the migration cost is high compared to the benefit gained from moving the virtual machine.
vMotion is not enabled or set up for the hosts in the cluster.
Address the problem that is causing the load imbalance.