We've investigated issues where customers running Ubuntu 12.04 do not see logins on their Dashboard in the Active Login Sessions widget. The resolution is to edit, or if needed add, the following line in your /etc/pam.d/sshd file:
session required pam_loginuid.so
The Threat Stack Agent relies on loginuid to associate users, logins/logouts and sessions and track active logins.
After you update the /etc/pam.d/sshd file, you may need to restart sshd and the Threat Stack agent.