S.L.
My dd had the same problem and I found out this is very common because they feel like a piece of themselves is falling out.
I put the pull ups on the back of the toilet, I had my dd put them on herself to go. I would help her take them off.
I also put a bag of toys on the back of the toilet (I wrapped them - just little trinkets). I told her when she pooped in the toilet, she would get a toy every time.
Finally she went and got the rewards until they ran out.