Having trouble with my 4yr old girl using the toilet

I have a 4yr old girl that refuses to poo on the toilet she will rarely do it on the toilet and when she does we reward her and give her heaps of praise but she continues to poo in her knickers and i dnt know why... she has been doing this for the last 12mths and i an at my wits ends we have tried everything we can think of to put a stop to this.. the things we have tried are : rewards when she does and consequences when she doesnt like taking her toys away, making her sit in the corner for 5mins, placing her on the toilet for 10mins every 2hrs, making her wash out her knickers and none of this has helped to solve the problem.... We have seen the doctor and they have said that there is no medical reason for this... I really need some help i think 12mths is enough and she needs to get this sorted as she is now at kindy.
I am open to any ideas on what to do...