It runs (depending on your OS ?) 8 minutes after boot. So it isn't running all the time, if you use the Simple User Interface to do an on-access scan, the Standard and Thorough sensitivity scans also do a rootkit scan.
What are the errors/warnings from the log viewer ?
The anti-rootkit log is here C:\Program Files\Alwil Software\Avast4\DATA\log\aswAr.log, it only retains information on the last scan and it only takes a few seconds to run.