One of my machines does not appear in the Machine Summary

A machine may not appear in the Machine Summary for a number of reasons:

The machine has not been restarted after installation.
Sufficient time has not elapsed since installing/restarting the machine (it can take up to 30 minutes after installation/restart for a machine to be added to the Machine Summary page).
There is no Web Sites Visited, Web Blocking or Instant Messenger data yet for the machine for the Client to send (there must be Web Sites Visited, Web Blocking or Instant Messenger data for a machine for it to be displayed in the Machine Summary).
The Client may not be communicating due to a network firewall/proxy issue (see How do I configure my firewall for Netintelligence Web Filter).
The Client may not be communicating due to a personal firewall issue.