Hello. recently we rescued a 5 year old shitzu. Her owner had died, she was alone in a house for about 3 weeks with a cat. She was in bad shape and dehydrated with some eye damage (which is healing up with daily drops), and had to go on an IV for a while. This was about four months ago, and she ended up in our home. We love her, her behaviour is great, she is well trained, quiet, and very obedient. She is a bit timid around strangers, but we think she'll begin to get over that.
So when we first brought her home, she was pooping and peeing in our fenced-in yard. Once early on i caught her eating her poop and i chased her away from it. At her temporary foster home she also used the yard to go the bathroom. She had been an apartment dog before this. But after a few days when we got into a regular walking routine with her, she began to not go in the yard anymore.
She went plenty in the yard at first, but after she saw we would also walk her that stopped. Sometimes i'll open the door to let her out into the yard, when i'm out there i'll take her, and she'll gladly go, but only leaves the back deck if she's chasing something or following me. Even then, she rarely leaves the walkway and steps foot on the grass! We've tried putting her on a leash and walking her through the yard, but she fights us when we try to pull her off the backyard walk way! So when i force her off the walk way into the yard, on the leash, she still won't go. Normally her walking behavioiur is perfect, we only have a problem in the yard. In our yard, she'll stay on the walkway and wont step foot off it unless a bird flies by or something. But the main thing is no matter what she NEVER 

ees or poops in the yard. By the way, walking on dirt or grass is not a problem for her on walks or at the park, etc, only in the yard!
We love to walk her, but we would also love it if she would use the yard once in a while, not only because we bought a house with a fenced in yard so that our dog could enjoy it, but for those early morning wake-ups!
thanks