Two notes of thought ...
1.) Both of my twins had this problem - they became real pros about holding until nap/bed time. I finally decided to reward with candy. I felt guilty about it, like I was bribing them, until it started working. I bought Skittles and a special see-through jar and placed it on the counter. This was candy they had never had before, but it was bright and colorful and had them intrigued. I explained to them that if they were big enough to make wet in the potty, they could also poop. I told them that I was not going to clean up poop anymore, that if they pooped in their underwear or pull-ups, they'd have to help clean it up. I also empathized with them that I understood it may be difficult, i.e. a new sensation, to learn how to poop sitting down. I assured them it would take practice. I also let them know that if they pooped in the potty, or even *tried*, they would receive a candy reward. And they could pick two colors themselves. Within a week, both twins were pooping in the potty. My only regret is that they now refer to Skittles as "poop candy," completely unabashed!!
2.) Some children, especially those who are capable of pooping on the potty (as your daughter demonstrated at age 2), may be holding poop for other reasons. I know it might sound unimaginable, but there is a certain pleasurable sensation in holding a bowel movement. I can attest to this personally because I did this for two years when I was a kid ... from ages 6 - 8. It's embarassing to admit, yes. And when I think back on it, I suspect I did it for the sensation (it felt better to hold than to go) and also for control ... my family had recently moved to a new community, life was new and exciting and there were many changes that I could not control. I guess maybe (?) that holding bowel movements gave me a sense of control. Anyway, I just suggest not ruling it out as a possibility ... some kids won't be bothered to sit long enough to go, to be distrupted enough to go, to deal with the clean-up ... so perhaps dialogue with your little one and find out how she feels about that aspect of potty training, and maybe there's something you could work on together to make the process a little less scary or daunting for her!. :-)