I have had that problem also, that when you type in the displayname of a user in the "Search By User" location, you must type something into the search input field also.
I then just decided to not put nothing in the "Search By User" location and just include the display name in the search input field, and it does show back all the posts that user's display name was involved in.
I dont know if there is any other easy way.
Not really an accurate way to do it.. Brings up posts by other members mentioning that member in their posts.. In the release notes for 4.2.5 it is mentioned that this is fixed...