Per default, cron logging in Ubuntu 14.04 LTS is rather limited. Standard logging in syslog does not offer any details if root-owned cron jobs are actually running and if not, why. Here you will see how to enable cron logging which will help you in further troubleshooting.
Edit 50-default.conf file and uncomment cron line:\n
# vim /etc/rsyslog.d/50-default.conf
remove # from the following line:
I recommend you restart your server, but restarting rsyslog service should be enough. So do either service rsyslog restart or plain old good reboot.