First let me just say that this is very very common. My daughter did the same until she was 3 ½.
There are various tracks you can take on this. One is that when she poops in a diaper/panties, that she watches you take it to the toilet and flush it. This way she begins to realize that it is all going to the same place – regardless of if she does it in diaper/panties.
Another tack is a reward system. Set up a “potty chart” and if she goes 1 week without having an accident in her diaper/panties she gets a prize. Go ahead a purchase the prize so that she can see it. It may give her an incentive.
A friend of mine a great success with that book “Everybody Goes Poo.”
I have heard that there is a physiological issue going on here. That some kids don’t like to go poop on the toilet because they kinda feel like they are flushing part of themselves down. I am not sure if this is true or not.
For us though it was that my husband and I didn’t realize that we were sending mixed signals about the potty. He tends to cave and not realize it. It wasn’t until I sent him out to buy pull-ups one day and he realized how much money we spending on diapers that things changed. He came home without the diapers and explained to our daughter that it was time that she started using the potty for everything and that there were no more diapers that she finally started going poop on the toilet. It only took about a 2 days for the message to really get through to her that Daddy meant business.
Hang in there. She will get it when she is ready.