do you mean not passing stool? or passing stool but it looks undigested? if the former, try soaking her in warm water. if the latter try raising the temps a bit. if that doesn't work take her to the vet. some more info would help. what are the temps? what is the substrate? what size enclosure?
I have two geckos. The male is fine, but the female is passing stools that look just like what went in. She is a bag of bones, even though she is eating. This started when she was in with the male, and I thought he might be bullying her for food, so I separated them. Since then she eats, and then poops it out in nearly the exact same condition. Meanwhile, the male, who is in a different tank, but has the exact same conditions is HUGE! Her substrate is digestable, and she has plenty of water. Any ideas?