I know it's cold, but have you tried going pants free entirely? It seemed to do the trick for my DD. Stickers on the calendar for each day she did a poo in the toilet with 3 stickers equalling a chocolate (or whatever might motivate your DD).
We used sticker charts with her in the beginning and we got success. At the moment she has a Dora scooter sitting on a shelf waiting for her to do poo's. Everytime she asks about it, and why she cant get it down, we explain its because she has to use the loo for poo's too. The scooter is what she wanted in return for tting, and I had to make sure she understood it has to be both poo's and wee's.
It will happen, just sooner would be better
With my DS I would physically put him on the toilet every hour and practise pushing noises and read a book 'everybody poo's' then if nothing was happening take him off 5-10 min depending how long until he was over it. After three days of this it all clicked goodluck
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