Recently, I have been having problems doing multi-city searches myself on Delta.com. I finally had to book through an agent on the phone. She told me that if you have more than 3 flights in a leg, then the search engine goes bonkers. Didn't use to but it does now. Not sure why, but it's a pain. Especially since I would assume that DL would prefer its customers to book online vs tying up an agent on the phone for an hour.