Search results are always limited to 200 items (posts or threads). If you want something other than the 200 most recent (which is the default), you can use Advanced Search to specify particular keywords, date ranges, sort orders, and/or forums to search in.

The perfect search would give you exactly one hit, and that would be the exact post you were looking for. Of course if you improved your search just a tiny bit beyond that point, you might not get any hits at all.