If you are running the version 3+ of rsyslog then you may not be logging iptables kernel logs.

This will prevent investigation of iptables block issues and the csf Port Scan Tracking feature.

You need to check /etc/rsyslog.conf and ensure that the following line appears near the top of the configuration file:

$ModLoad imklog

If you have added that line, then you must then restart rsyslogd:

service rsyslog restart