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
Select Tools Menu item
Select Instant Search submenu
Select Search Options
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.