Full scan fails to start on schedule

I have a full scan scheduled to start once a week, but it fails to start on schedule. This is on my HP Pavilion g6 laptop. My husband has two laptops with Avast! installed and a weekly scheduled full scan. These scans always start automatically on the first available opportunity after the scan is scheduled, i.e., as soon as the laptops are switched on. We’ve both checked the schedule and can’t see any reason why the scan doesn’t start automatically. It does start manually. Does anyone have any suggestions as to why the scan isn’t starting automatically or anything further we should check?