No result when searching in a shared mailbox / All Mailboxes
When you have problem searching in a shared mailbox you could try on of the options below:
Do you have the latest Office Updates? There was an issue with Outlook 16.0.8827.2062
Check the Cache Period settings. Do you have set it to all or only a couple of months?
Try without 'Download Shared Folders'
-> "More Settings"
-> "Advanced" tab
UNtick the "Download shared folders" box
restart Outlook, are you able to see more results?
This document gives you information about the Search Scope. When can you only search Current Folder, when can you search All Subfolders, when is the search done by Exchange, when is the search done by WDS:
Note: The article also applies for the latest Outlook versions
Auto Mapping: If it is added through auto mapping, by default you will only find results if you select "current folder". Try adding mailbox without Auto Mapping (manually).