so as a leader

I'm a little frustrated with one mom in our troop. Its so hard to be a good leader and do something that challanges all the girls but also so that they enjoy it and have fun. I spend TONS of hours finding new and exciting things for the girls to do and learn. And its really frustrating when one person is so vocal about her dislike/distain for everything we do in our group. Do people not realize that I'm a volunteer and I have other things I could be doing with the hours I spend working on scouts and programming. I have a large troop of now 21 girls and this year we've done a few field trips, camping and alot of meetings. We've done community service projects, fall sales (magazines, nuts and calanders) which I ended up being the product manager for and now we are doing cookies for which I'm the cookie mom. I'm tired! My son wants me to do stuff with him! And I have a life. This same mom has failed to do anything she has said she would do which is making me even more angry about it. There is nothing more irritating (or at least things rank up there with it) then me having to run around at the last minute picking up balls other people drop. So now I have to figure out how to handle this ..especially since her daughter is such a sweetheart and we love having her in the group. Anyone got any hints/suggestions?