Sometimes a flight search for a one way flight from A to B shows tons of results, and B to C shows tons of results too, but if you do a multi-city search from A to B and B to C, you might not see any results. The doesn’t sound intuitive, but there's an explanation for the behavior -- speed. Broad searches that require the comparison of many itineraries are inherently slower, while more narrow searches can be deeper in the same amount of time and therefore return better searches. We are always striving to improve this behavior, but there is always a trade-off between search depth and search time.
Also, airlines file rules that limit our ability to connect flights into itineraries. It might not be possible to purchase the flights from A to B to C on a single ticket. Additionally, flight availability is constantly changing, so you may see slight variations in the results you get when repeating your search.