How can one fix a Mac OS App Store that stops updating?
In addition to the auto update issue, I noticed by chance that the App Store showed download arrows for iWork apps -- when they were clearly installed on my MBP’s internal flash drive. I had the insight that something might be wrong with Spotlight. After I rebuilt the Spotlight index, not only did the App Store properly recognize that I had the iWork apps installed, but automatic updating appears to have been restored.

So it’s obvious that the AppStore relies on Spotlight for its update mechanism.