If the situation does not perform regular tasks crontab appears, you first need to locate the problem, then you need to identify the problem through the log.
position generally located crontab log / var / log / cron, using the following statement to view the log.
tail -f /var/log/cron
/ Var / log / cron script will record whether certain programs, if failed, unable to give further information on the error, then an error message that we will need to execute the statement
Redirected to a file, so you can easily view the error message