I think I only had very very few people say no. Mostly because they were in a rush for something (Panel, costume judging, etc). With at least an excuse, it didn't seem like much of a big deal. I understand timing and schedules. A nice thing I have people do instead of saying no is say not right now but later. "Oh we are actually about to eat / I am heading back to the room to freshen up and relax if you could catch me later then I would love to pose for a picture"
Message me for Photoshoot scheduling and information for any con I am attending.