By default, DRS only kicks in, if any DRS rules are violated, or in case of resource contention.
As of vSphere 6.5, DRS comes with an additional option to balance VMs across hosts.
Run a test by performing a vMotion. If that goes through, you know you dont have any configuration issues. What's the cluster utilization looking like? Select the cluster and in the summary page, check vSphere DRS status. What is it saying? Go to Monitor tab --> vSphere DRS. What is DRS recommendation showing as? You can run DRS manually as well from this page.