This has happened to me before, but on a more intimate scale. I guess what I tried to keep in mind when it happened is that while I know a good deal of people who would have held high interest and asked me more, but somewhere between the audience, my spot in the lineup, and the success of the other presenters just happened to outweigh what I could bring to the table. "I am enough, I have enough, I do enough".
My father was once on a panel discussing drugs and alcohol to the parents of an incoming high school class. The members were a lawyer, a former alcoholic/drug addict, and him (a psychologist working largely with middle and high school aged students). When it came time for questions, every single person was questioning the lawyer on "But what if a kid gets drunk and injured on MY property if I wasn't home and he started drinking at another house and he had his parents permission on a tuesday when mercury is in retrograde???". Not a single person asked him a question, and it became extremely apparent that almost no one in the crowd understood the point of the presentation. Even in a group of 250+ people, there can be a single fixation I guess.