The problem is (at least partially) with the fact that the search facility only looks through the posts in the section you're currently viewing (or last used). So, for example, I might see a question in 'Technology' and realise that there were some excellent answers to a similar question some time back. However, when I search for that question, it doesn't show up because it was posted under 'How it Works', and not 'Technology'. It would be far better if the search facility always examined the whole of AB, not just part of it.
BTW, I've found a way of getting round the 'topic specific' search problem. I normally use Firefox and log in automatically onto AB. Even if I open a new tab and go to AB's home page, the search only looks at the topic I'm currently viewing on the first tab. My solution is to open Internet Explorer as well and go to AB's homepage. (I've not set up IE to log me in automatically, so I'm only recognised as a guest). Then any search examines the whole of AB and 'missing' questions will show up.
Chris