Scheduled scan task inconsistent

I've been trying to get the scheduled scan task to work (on Windows 7 running version 22.5.5.15)  reliably and consistently without much luck. I've set up a task to wake my laptop at a certain time so that the scheduled scan can run.  What I experience is that the scan either does not run, or, the time I entered for the scan has somehow been changed without me doing it.  Does anyone know what's causing this and how to fix it?