DD is a couple of weeks away from turning 3 and has basically been ready to use the potty for ages.
We made an abortive attempt over the summer, but she started withholding her poos and then crying as it was sore when they eventually came out, so we figured she wasn't quite ready, and waited a few more months.
When wearing a nappy (or pull up) she will hide behind a curtain etc. and go for a poo. She absolutely knows when they're coming.
So we have gone cold-turkey (except for bedtime and naptime) and put her in knickers all day. Wees are OK. She will generally go if we remind her and put her on the potty.
However, when she needs a poo, she will tell us, but then cry when we put her on the potty (or toilet, with a special seat - she has the choice), and basically (gross, sorry), suck it back up again so it doesn't come out. Then she has withheld it for so long, it becomes painful for her to pass.
When we change her into a nappy in time for bed... out comes the poo (with some more crying as it is genuinely painful by this point).
When she needs a poo, she also withholds her wee.
She eats brilliantly and healthily - lots of fresh fruit and fibre. Poo is never a problem when we are using nappies.
I have tried sticker charts, bribery, reading books about it, talking about it, watching her big sister do it... nothing is working.
I suggested putting a nappy inside the potty for her to poo on but she laughed and said that was disgusting...
Any and all advice gratefully appreciated.