J.S.
Have you tried using the big potty at the same time as her. If she doesn't feel like all of the attention is on her then maybe she will. Even if you are not using the bathroom, you can pretend and it may make her feel more comfortable. Find a book to read or do a crossword to keep from occupied. I think all kids go through that where they are scared for some reason to poop. Face it nobody wants to poop with someone standing over them watching their every move.