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:Please mark my comment as the Correct Answer if this solution resolved your problem
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.