rebeccachor,
My daughter also had this problem before. She could not NgNg for 4 days. NgNg was very hard and she cried. We need to bring her to see a doctor.
But now, she can have NgNg everyday. (actually, she used to have it twice day per time)
The reason she can have this improvement is that I let her eat many vegetables with congee. (such as pumpkin, corn, 瓜, 菜, apple, etc). It really works you can try and see any improvement for your baby girl. Of course, it need to take 1-2 weeks to see this improvement.