The search function is working. The newest threads have priority over older threads in the search engine. I’ll admit, a lot of the time the search bar comes up with unhelpful results, but it is working. And another thing: bigger threads with more posters will have more words, which means a higher possibility that you will search one of those words, coming up with results you don’t want.
Notice when you search, there are two different categories: recent threads, or related threads. The recent threads may or may not have as much connection to your question as the related threads.