Sigh, chicken logic. I understand what you mean about her being an unsteady setter.The mom was not very good. She would get up and go to a different nest every couple days and I would move the eggs under her. Then, obviously she gave up. I waited too long to put the eggs under her, so it was completely my fault.
I had two hens go broody this spring. One Dominique and a Dom/Buff cross. The Dom/Buff was in the same nest, all day, every day and wouldn't move even when I got the eggs from under her or when other hens would lay eggs on top of her. She would growl, but never offered to bite. The Dominique, on the other hand, was what I call "crazy broody" she would hop from nest-to-nest, even tried evicting the Dom/Buff from her nest, and she was MEAN. She kept trying to take chunks out of my hand whenever I tried to get eggs from under her. I got really good at grabbing her hackle feathers behind her head to keep her from biting me. We decided to give the good broody some eggs (even though we didn't particularly want chicks from the only mature rooster we had at the time) just to test her out and we decided to bread the crazy girl. The good broody was an excellent mama. While she was hard to break, we finally broke crazy girl and any time she shows signs of being broody I put her in the breaker.