S.M.
the only thing that worked for me was a really big bribe! I had three boys and one only went for the bribe, I threatened to make the third one clean his own undies if he pooped in them and the first one didn't have an issue with it.
if she has a tendency to poop at the same time each day you might try just sitting her on the potty for 20 minutes and read a book, but that didn't work for me, but has worked for some of my friends.
with my littlest one he did a lot better if he could hug me while on the potty, he would grab on to my legs and lean way over and hug, I think he was afraid of falling in when he was pushing out the poop, and being bent over seemed to help a lot, he also always had to have his knees up high so we used a step stool under his feet so that helped also, but the hugging part was very important to him, sometimes I sat on the step stool in front of him and he could hug my neck, but being bent over and not afraid to fall in helped, we never used a potty chair with any of my kids.