Avast rootkit is not working?

Hi Experts,
I am having the Avast anti virus and tried to scan my PC with Avast rootkit to see there any rootkit embedded to the softwares I have installed. But nothing detected. Then I try another rootkit checker and it worked. I really want to know why my Avast rootkit does not work. Advise me.
Regards,
Shirlyn Winchel
