On the client computer that does not appear in the ERA Web Console:
On the ESET Remote Administrator Server:
If the responses from nslookup for the hostname and IP address do not match, there is a problem with your Forward DNS and/or Reverse DNS zones. See the following Microsoft Knowledge Base Article for information about troubleshooting DNS Zones. DNS changes may take up to 12 hours to take effect on your network and be reflected in ERA Web Console. Check to see if missing clients are found after the changes are in place.
If you are still unable to resolve your issue, email ESET Technical Support.