I don’t know what’s going on with Outlook’s Indexing, but it would miss tasks – even when I could see them by scrolling down by date. I had enough of it and disabled indexing. This forces Outlook to go back to the slow search method that actually works.

Here’s how you can do it

  1. Select Tools Menu item
  2. Select Instant Search submenu
  3. Select Search Options
  4. Uncheck the Data file that is missing stuff (or all of them, if you don’t know)

Hopefully MS will fix that painful issue. However, I don’t see a good fix in sight, even after a little digging.