How can one fix a Mac OS App Store that stops updating?

I've been occasionally (but annoyingly) noticing a situation with Mojave (I've never seen it in OS versions before Mojave) where I know that an app purchased through the AppStore has an update available, but the AppStore will not show that the app needs to be updated. In order to get the update, I have to delete the app and then redownload it.

I first noticed this for Agenda. Then the problem went away for a while, and apps that I purchased through the AppStore updated automatically.

Now it is happening for Ulysses. Specifically, Ulysses had updated automatically to version 16 on my iPhone and iPad, and since it is always updated at the same time on all platforms, I launched the AppStore to check to see whether it had been updated on my MacBook Pro. The app is still at version 15.2. When I launch the AppStore, I see this:

So it was updated on the AppStore 16 hours ago. Notice how it shows "Open" in the top right of the screenshot? Well, if I click that, it does launch Ulysses -- version 15.2.

Solution 1:

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.