If you have a firewall between you 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 Web Client access, and firewall commands. See Incoming and Outgoing Firewall Ports for ESXi Hosts for a list of ports.