You can enable logging for cron jobs in order to track problems. You need to edit the /etc/rsyslog.conf or /etc/rsyslog.d/50-default.conf (on Ubuntu) file and make sure you have the following line uncommented or add it if it is missing:

cron.*                         /var/log/cron.log

Then restart rsyslog and cron:

sudo service rsyslog restart
sudo service cron restart

Cron jobs will log to /var/log/cron.log