Play the Battle of Osaka Castle (Chapter 3) and make sure Diao Chan is in your team. When Lu Bu appears after you clear out the bottom left base full of generals, meet with Lu Bu and he should become you're ally.
Did you listen in on a talk between Diao Chan and another person? Correct me if I'm wrong, but that may be the trigger needed to make him come around. Honestly though, I don't see how it's that hard. Unless you don't talk to everyone each time you're in camp then even this shouldn't be a mystery. Talks often trigger map unlocks and new events. It's not hard. Really. :/