If the user has not registered, like Allen Hatcher for instance, the MO user search requires an exact name match, partials are insufficient. For registered users like Cam McLeman, partial matches like doing a search on "Cam" will work.
A sometimes more convienient way to do a user search would be to do a search on Google. For example, this Google search string finds Hatcher's user page quickly:
I think the algorithm is that if there's too many matches -- I'm a "bu" match, for example -- they aren't displayed. Matches only get displayed once the time it takes to put them on the screen is sufficiently small.
I thought something similar, but if you type "BU" into the search bar, it's not that you get no results, it's that you get exactly one page of results (that does not include Bugs Bunny). I guess the algorithm is to put forth a minimal effort to find matches until the search parameter is more precise?
Regardless, the whole system is a little buggy. For example, if I search for "as" (a random 2-letter pair I thought for sure would have some results) reports back to me 2 pages of results, but clicking on the second page gives me the second page of *all* users, not a 2nd page of "as" results.