An interesting incident happened on the server after a global update CentOS 7.1.1503 to version 7.2.1511. Usually when you upgrade CentOS anything happened with SELinux. Somewhere something changed contexts somewhere else something. But in this case, after you upgrade CentOS, daemon logging syslog-ng stopped writing logs. Not all logs, and most of that was getting through syslog.
As it turned out, the system logging ceased publishing send copies of logs to syslog. But the fix is quite easy, to do this, run the following commands:
$ nano -w /etc/systemd/journald.conf
$ systemctl restart systemd-journald.service
$ systemctl restart syslog-ng.service