The below links to an excellent article that covers not only dealing with Outlook finding no or incomplete results when the user is doing a search, but is also educational about how search works in general. For example, I never realized that the .pst file is the extension for online data files, whereas the .ost extension is the one Outlook creates for offline data storage, something that is configurable.
https://support.microsoft.com/en-us/help/2769651/outlook-search-returns-no-matches-found