Oh boy!! I am going through the exact same thing right now with my daughter, Kayla, who will turn 4 in October. She'll pee just fine in the potty but won't poop. I keep asking her why she doesn't want to poop in the potty. She tells me: "But I did poop in the potty." Yes, that was just one time, a year ago!
We also are switching between underwear and pull ups. She is not confused about that. She'll poop in either one. Rarely has a pee accident. Lately, she's been wearing underwear every day, and most nights.
Kayla sounds just like your daughter. She's a very independent, stubborn, head strong little girl. She never cries when she gets hurt. She'll insist very stubbornly on something for 4 or 5 minutes ("I want daddy to get my snack, not you!") and then just when you're at your wits end, she'll turn on her smile and change her mind, as if it was her idea in the first place.
I wish I could offer advice, but it sounds like I need it too!
Kayla goes to our local YMCA full time for pre-school. They are just as vexed with Kayla as I am. They say she's one of the more stubborn ones they've seen.
We keep telling her that she won't be able to go swimming at camp this summer in the pool with the froggy slide. (Kids who aren't potty trained can only go in the blow up, splash pool at the Y.) She wants to go in the play room at Wegmans (grocery store) like her big brother but they won't take her until she's trained...so I've been trying to offer that up as incentive. Stickers, potty charts, candy (yes, I've tried sugar-free lollipops!!), cake, ice cream, nothing works.
I am not being pushy though and never get upset when she has an accident. "That's okay, it's just an accident." I wonder if I should get a little firmer?
I am at such a loss with this kid. My firstborn, a son, woke up one day when he was 2 yrs, 3 mos, and was completely trained. Just like that. Not one accident. We never even did pull ups with him. I guess this is my payback for having such an easy time the first time around.