I have found that searching for user names in the general search spot (at the bottom of the forum page) is not successful. It seems to search the body of the texts posted, not the user names.
If you want to search for posts by username, there is a separate function to use. You go to "member list" at the top of the page, and then can search through alphanumerically until you find the user name on whose posts you want to do the search. Then if they have posted, there will be a place next to their name with a search button. Use that and you get all the posts for a particular user.
It can be a bit trying finding the user name, because of the huge number of users, and the bizarre names with numbers and symbols inserted etc making the normal alphabetical search a bit harder, because it is really an alphanumerical with symbols list.
I don't know the reasons, that's just the way I've found it best to do the searches.