If your DRS cluster is set to partially automated or manual mode, no migrations will be performed automatically by the DRS algorithm. You have to approve the recommendations manually.

You can check what recommendations are available in the Monitor tab of the DRS enabled cluster under vSphere DRS and Recommendations:

You can either apply all recommendations using the APPLY RECOMMENDATIONS button, or you can override them by selecting Override DRS recommendations and then select what recommendations you would like to apply.

Once you apply the DRS recommendation, vMotion will be issued to move the VMs between ESXi hypervisors.

In DRS history section, you can find what migrations were performed by DRS and when were they performed.