As a first step in troubleshooting I would review the configuration of the Cisco device and also the ACS server configuration. If you post the configurations we could also assist in reviewing them. I would check the configuration of the Cisco device and verify that the server is correctly identified and is configured with the correct shared secret key.
As a second step in troubleshooting I would verify connectivity between the Cisco device and the ACS server. ping and traceroute would be tools to use for this.
As a third step in troubleshooting I would check the ACS server logs. Look to see whether the server indicates that an authentidcation request was received and if so what did the server do with it. Are there any error indications in the server logs?
HTH
Rick
HTH
Rick
