The admin made a change to the search system several months ago (I think it was over a year ago). It no longer shows users when you search, only games.
If the user you're looking for has indexed games, it will show you those; if they don't have any indexed games, it won't show you anything.
If you know the nickname, you can always use the profile URL, which has the following format:
itch.io/profile/<profile_name>