Try feeding her bran cereal (with NO artificial sweeteners listed in the ingredients!) softened in plenty of water an hour B4 a walk. This has helped me correct crate/indoor pooping issues with several puppy mill fosters by increasing the urge to eliminate; They poop outdoors, giving me a behaviour to reinforce. She might be a shy pooper, too–if you can, find her a screen of tall grass or brush for privacy, don't interupt her with praise or be loud offering it, and never yell or strike her for pooping indoors...many dogs won't potty within sight of their owners for fear of being reprimanded.