Hi,
The searching in Outlook is affected by the search conditions. Before the search you can choose if you want to display the serach results from current folder, all folders or current mailbox:
You probably can't find the emails from archive in serach results beacuse of this setting. To include archived emails into search results, try to select Current mailbox or All Outlook items option. If there will be no emails found from the archive, try to click on archive folder and then run the search again.