When viewing the datastores that are connected to an ESXi host within the vSphere Client, the local datastore and possibly other datastores connected to the host are not displayed.
This issue is caused by a host being in a "Not responding" state, which is caused by the vCenter Server not receiving UDP port 902 heartbeat traffic from the host.
Ensure that a firewall or other network device is not blocking or interfering with UDP 902 traffic between an ESXi host and the vCenter Server.
Please see KB article 318895 for ESXi port requirements: