Hi, we went thru this with our little girl and a sticker chart worked for us! She loved putting a sticker on her "poo chart" when she's been for a number 2 on the potty or toilet and with lots of praise, within a week she cracked it. Maybe something like that would work for your little girl? X
We had poo accidents for about 4 - 6 weeks then it clicked. There's a you tube video called "poo goes to poo land" my girls loved it not sure if repetitive watching of that worked or they just got it but poo does take time as it's a different feeling in your nappy to the toilet. Good luck.
This is totally normally, and my little girl did the same thing, except she doesn't wear a nappy at all so she would go days without a poo. What I found was having a box of toys and books for the toilet only, meant she'd stay on the toilet long enough to do a poo, and then a lot of praise afterwards
We are in a similar position - occasionally able to get her to sit on the loo/potty to poo but mostly nappy in the morning.