Potty Training -part 2

I had been trying for months but my girl still don't seem to be able to shit on her potty. When she show the face that she wanted to shit, i will bring her the potty and she is willing to sit on top of it. However she cant sit on it for long. Soon she will ask me to bring her wet tissues boxes and start playing with it until she forgot about the bowel movement. If i dont let her play the wet tissues, she refused to sit on the potty. Had tried many time and it seems that she really can't discharge when she is sitting down. She must stand up and shit. Well, I think she might need somemore time to get use to it. I was hoping that one fine day, she will suddenly able to shit on her potty :)