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:
# 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.