I have a cluster with 5 nodes. first three nodes has 768GB RAM each and last two has 1.5TB RAM each. CPU count on all hosts are the same.
DRS is fully automated but I'm seeing last two nodes have plenty of memory available (50%) whereas first three nodes goes upto 90% utilization. I think DRS is not balancing VMs across nodes in this cluster. Is there a requirement where all nodes should have equal amount of memory for DRS to work\balance correctly?
Did you try to enable and disable the DRS feature in your cluster and check the result again? What happened if you start a new or power off VM inside your cluster?
It's not bad to check your vCenter Server log files that are related to the cluster settings. You can find the path of related log files in the following link:
Which version of vSphere is this? Do you have any advanced DRS settings configured? Note that DRS doesn't focus too much on balance, but more on VM happiness, so if the VMs are getting the resources they are requesting there may not be a reason to balance and move VMs.