Potty Training?

Okay I am in need of some help. I have a 2 1/2 year old girl who does not want to potty. I have tried just undies and she still refues I have sat her on the potty for 45 minutes plus and no results i have done the every 15-20 minute things and still nothing I have tried treats can anyone offer any advice on this. My son who is now 8 was so easy they always told me boys where harder but not true I watch a 2 year old boy during the week and I have him pretty much trained and i do the same thing with tem both he picked it up right away. please help. thanks in advance for you comments