I don't use Outlook but I did have the same problems with indexing that many others mention.
My solution is to close everything that is open and Restart.
When has started, then type 'indexing' into settings or control panel and open.
Then you will see the indexing take place by the file count increasing.
Don't interrupt by doing anything else (including allowing your PC to sleep or hybernate) until it says it has completed.
Repeat this before every time you want to do a search.
I am a 70 y.o. computer user, never trained in IT. I scratched my head about the search results many times before I stumbled on this solution, as ridiculous as it is.
From DOS through to Windows 10, indexing has never been a problem. Windows XP had the best search features.
Under Windows 11, even searching by a range of dates is complex. Computers are supposed to save us work and frustration.
Why does Microsoft remove wonderfully useful and satisfying features every time it puts out a new Windows?