Idle Full System Scan Doesn't Resume

NIS 2011, Windows XP-SP3.  I have my idle timeout set to 10 minutes.

An idle full scan was underway today when I had to briefly open an MS-Word document to check something, so the scan stopped.  Then I left the computer and did not return for two hours.  When I returned, the log did show -- as expected -- that scan was canceled when I looked at the Word document.  However, the scan did not resume.

It seems like I've been having this issue for 3-4 weeks now.  Namely, when an idle full scan is interrupted it doesn't resume as it used to.  I've even waited 3-4 days for resumption, but it doesn't retry until it's time for the next weekly idle full scan.

Any thoughts or suggestions?  (For reasons I won't go into here, I prefer not to upgrade to 2012 right now.)  Thanks.