I think you are right to pet her and give her verbal praise at every opportunity, as long as she's not afraid of being touched. If she's okay with the touch, gentle, loving pets are a good way to remind her that nothing hurtful comes from your hand.
It sounds like your Yannah didn't have humans she could trust in her past. She'll likely take more patience and creativity than your other dogs, but I've never had a more rewarding experience with a dog than I have seeing the timid ones come out of their shell and find their self-confidence. It's an amazing, life-altering relationship with a dog, when it works.
I don't know how you trained your other two, but with this kind of dog, positive methods are very, very important. She's the way she is because someone probably screwed up the "corrections" with her in a major way. If you decide to hire a pro trainer, please look for someone who uses positive methods, who has had success with shy dogs.
The secretive "potty" thing you describe is what happens when someone scolds or punishes a dog for going in the wrong place -- Yannah likely internalized the punishment to mean "humans get mad when they see me potty" (not "don't potty in the house" or whatever the human intended). You are, unfortunately, suffering from the potty-training correction errors of a prior owner.
Now she's afraid to let you see her do it and will be secretive...so how do you praise her when she does it, when she won't do it in front of you? The only way I can think of to undo this is to be sneaky and watch her without her knowing you are there, outside--the timing is tough to get right, though, as you need to praise as she's doing it, and watching from your kitchen window will make the praise too late. The goal is to let her know it makes you the happiest human who ever lived to see her potty outside.
If she's comfortable walking with your other trained dogs at this point (it might be too soon--I don't know), you can consider enlisting them to help. Tell them to potty on the leash, then treat and praise effusively. She'll notice. Same with in your backyard. They may be able to show her that it's okay to trust you, and through them, you can show her you don't get mad when you see dogs potty in front of you.
The food thing suggests she's also been punished for taking food she wasn't supposed to take, or taking it to roughly when she was young, so she thinks taking all food from humans will make you mad and result in punishment. Again, let her watch you with your other dogs. When she sneaks back in to take her meal, praise her (I wouldn't touch her while she's eating, yet -- happy, delighted, verbal praise is good here--the clicker trainers here might have better suggestions using that method with her, as it might make it easier to communicate with Yannah). Feeding her in a crate as part of crate training may also help her to realize that (1) she has a safe space to eat, and (2) it's okay to eat in her safe space.
I know nothing about Schutzhund other than watching events a few times, but I've rehabbed a few shy rescue dogs to become very good pets, and the one piece of advice I can give that should animate everything you do with this dog is (1) build her confidence, and (2) give her a reason to trust you completely and do nothing that would undermine that trust. Do everything you can to set her up for success and be effusive in your praise and rewards. Praise her courage and bravery when she does little things that show a bit of confidence.
She sounds fixable, and I salute you for trying to help this girl! BTW, I've always wondered whether one of these timid dogs could be rehabilitated to have the kind of self-confidence needed to succeed at Schutzhund -- if you do it, will you please post about your journey? I'd love to sit on the virtual side-lines and cheer Yannah's success!
Thank you for the encouraging words! I trained my other two using clicker and N.I.L.I.F im no pro or anything but they are very well trained. I took them to one session but we never returned. In my past experience with trainers and or breeders they seem to be... ** comments removed by ADMIN. Swearing, breeder bashing and attacks on people is not allowed on this board. **
Anyway back to Yanah, shes Getting better with the food. this morn i sat her bowl at the ground and had a little ground beef in my hand positioned right over the bowl she took it and i praised her good, i will probally stick to this.
as far as the potty situation i took her on an hour and a half walk and nothing, she sniffs and walks in a circle like shes going to and nothing, bring her in and she of course poops, i cant let her stay outside with Semi & Germ because shes in heat and her area isnt done yet(cement still drying)
but i will use your method to let her see its ok to eliminate in front of me!
as far as Schutzhund ive got my feet wet but im thinking im ready to jump in I love the dogs so much and im always working with them so why not.