If a log in your Snort-log directory is an empty file like this:
-rw-------. 1 root root 0 Sep 20 11:30 merged.log.1411187404
-rw-------. 1 root root 0 Sep 20 11:30 tcpdump.log.1411187404
One of the causes of this problem may be the “$NO_PACKET_LOG” option (-N) in Snort’s startup script at /etc/init.d/snort. Please delete it from this line:
and restart snort again.