My little girl will not pooh on the potty...please help?

My little girl just turned 2 1/2 year old. She will pee on the potty but we can't get her to pooh on the potty. She will come tell you that she has poohed her pants just a little after she has done it. We ask her why she didn't pooh on the potty and she responds "I don't know" it's driving me crazy!! Any tips?