having nagios running, and upon the suggestion that it checks the services every 3 minutes, when I refresh the service status page every 10 seconds, sometimes I see all services in green (everything is fine), sometimes I don’t see all the hosts declared, and sometimes I see that some services change their status to unknown.
You have more that one nagios running. Either reboot your computer, or stop all nagios process’ and start it again.
Another thing that is likely, is that you made a change to a config file, but you never stopped nagios and restarted it.