One misconception many people have is that if they can't find a flight using a search engine or on the airlines website, then it must be full. As vacation travelers, people usually search months in advance, and flights just don't get full that far out (unless there is a major change in schedule and people are shuffled to the remaining flights). If the flight hasn't been removed from the schedule, then it's likely the result of the search inputs.
Most all flight searches automatically look for lowest possible fares. For instance, there may be 3 flights all at the same price, and a 4th $20 more (at a more convienent time), and it may not show up. Usually, you'll need to search using a specific time range to find the flight. If you still can't find it, call the airline directly and ask them if you can get a quote or make a reservation for that flight. They'll let you know what's going on.