We are 4 weeks into n75 with our 21m old girl. She is peeing in the potty with great success! However, she only wants to poop STANDING UP and is very resistant to sit on the potty when she has to poop. She will hold her poop all morning or wait until her nap (when she has a diaper on). We use the timer, gate and doctor. All of these contributed to her success with pee but not with poop. Any advice?