It indicate for a lost of connection, and possibly a problem with credentials for the percona-agent user to access MySQL.
Try restarting the agents on the nodes that show this behaviour by issuing:
root-shell> service percona-agent restart
or
root-shell> /etc/init.d/percona-agent restart
and then check their status again, they should appear as running after you restart them. Are you running latest version?
[url]https://github.com/percona/percona-agent#updating-the-agent[/url]
or