If you have a firewall between your ESXi hosts and vCenter Server, ensure that the managed hosts can receive data.

To configure a connection for receiving data, open ports for traffic from services such as vSphere High Availability, vMotion, and vSphere Fault Tolerance. See ESXi Firewall Configuration for a discussion of configuration files, vSphere Client access, and firewall commands. For a list of ports, see the VMware Ports and Protocols Tool™ at https://ports.vmware.com.