Disaster: She doesn’t care.
The deal: All-night DIY sessions are supposed to be fun, right? Well, kind of. But don't forget, she still may have plans on Saturday night that don't involve stuffing favor bags. To many bridesmaids, helping with planning is part of the fun and something they're totally up for doing -- granted you keep your sanity and a sunny attitude. Every once in a while, though, you'll get the attendant who wants nothing to do with early morning bridal salon runs and trips to the florist, and it can ruin the fun for you or, worse, for the other bridesmaids.
The solution: Explain how much it would mean to you, not just because you have 148 mint tins to fill, but because you want to spend time with her. And if she's too busy to come to every single appointment (and she will be), that's okay. Remember she has a job and a life and things to do besides worry about your wedding.