This is common. dullta.com and the res system the agents use will purposefully often not provide the shortest and most logical routing in O&D searches. It's like it on purposes suppresses the most direct, shortest distance and shortest flying time routes. It cannot be used (effectively, anyway) for O&D searches. You need to search segment by segment for all the possible routings to see the real availability. Then you need to get a good agent to put it together segment by segment and then price it out properly.