Check to see in your General Settings and Alerts if you have the (box) (unchecked). Where it says "Wait until your PC is idle to run sheduled tasks like scans,automatic updates,and Quick Clean. Periodically other programs like Windows Updates/Add Ons could be running at the time for your scan to start. Which can cause the start of your scan to be delayed.
Also make certain your clock has the correct time.
I hope this helps...