I have never been able to get that to work. It is ok when I do one for myself but I have tried it for a few others and it comes up empty.
If you are looking for all the posts a certain user has made, you must enter their username in the search box on the right hand side and not the keyword search on the left. If you enter a username in the keyword search box on the left, it will only search the body of messages for occurences of that name, which basically means the only results that will come up will be posts where that username was mentioned -- an entirely different result altogether. If you want to search for all the posts a certain user has made, the easiest way to do it is to click on the number of posts shown below their username in one of their posts.