MY GSD IS GROSSLY UNDERWEIGHT

Question
My bitch is 2years. I have had her since she was 7months. She was slightly underweight when we bought her from a breeder and has slowly been losing weight since. I have been to vet concerned about her very poor dietary intake and weight loss who has told me to stick with one brand/type of food. I have been doing this anyway and on the rare occasion when she has emptied her bowl I have rewarded her with praise and treats. She doesn't appear to enjoy anything you sit down to her but will eventually take half of the portion on 4th or 5th day. I have changed the type of food only twice in all the time I have had her so I don't think it is a 'hold out' to see if there is anything better on offer. Three months ago my mum looked after my dog when I went on holiday and she had put on weight on my return. She looked really healthy and full of energy but my mum had been feeding her dry mix mixed with leftover meat and fish from a care home that she worked in. She is back to being underweight again and has developed really dry skin and scabs are starting to appear with her scratching. She doesn't have flees. She drinks plenty water. I am now getting embarassed to walk her because other dog lovers and commenting on her weight and I fear they might think that I am starving her.What can I do apart from feed her 'chicken fillet and rump steak'?

Answer
How concerned is the vet?  Is she really thin, or do her ribs actually show?  If not, keep feeding a dry, meat based chow.  It they do show, try adding unsalted chicken or beef broth to her food.  

You need to be very careful about what else you give her.  Too much rich food isn't good for a dog.