Why crontab is not able to run some commands ?

[Halid Faith]

I have a script. As I am a root user,  I can run it without a problem. I
added that script to crontab in order to run as automatic.
I entered in /etc/crontab and put down as below;
*/20     *       *       *       *       root    /etc/scriptfile

Despite root user, the crontab could not run above file. But I can run the
same script in command line interface.
I am sure tha the crontab daemon is running. Also I can see the crontab try
to run that script in /var/log/cron.
/usr/sbin/cron[98727]: (root) CMD (/etc/scriptfile)
When I am a different user, I couldn't run the script and I got "access
denied"
What is the problem ?

Thanks