Is she "regular?" If so, try to sit her on the potty around the same time every day. If not, try something in her diet that'd help. If my guy has just one sprig of broccoli a day he will poop first thing in the a.m. which has greatly improved our quality of life.
If DD starts to poop in her dipe and you notice, move her to the toilet to finish it up. Point out the difference of how much better it is to go in the toilet, not get messy. Praise her for telling you about poop (before or after it's done). Get potty books at the library. Go at the same time she does-- and with all this encouragement, when SHE is ready, then she'll do it.