The search box above defaults to "board" which reallly just searches the sub-forum you are currently in. "Category" is the next step up and will search all the sub-forums in the current category. To search the entire forum select "Community". I think this is a bit confusing, I usually have to think about it when doing a search. (My way to remember which one to choose is that the default one is generally not the one I want :-).