Did you click Apply after setting it? Sorry these forums are patrolled mainly by us unpaid volunteers and we try to answer as many people as possible.
Also, are you doing this while logged into the machine as an Administrator?
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Please try the followng steps..
- Computer should be turned on and be idle without any running applications 20 minutes before the scheduled scanning.
- Check for any 3rd party scanning softwares and remove any if found.
- Disable Screensavers and if it is a laptop, keep it plugged in and disable powersaver options.
- If issue persists, try a complete re-installation of McAfee ( Use MCPR tool after removing McAfee from Add/Remove programs)
Please update with the outcome...