My percona-agent.log is full of it:
[MySQL] 2015/03/12 20:42:13 packets.go:118: write unix /var/lib/mysql/mysql.sock: broken pipe
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?
https://github.com/percona/percona-agent#updating-the-agent
or