Why is my boxer malnourished?

I have a boxer that I got as a puppy when she was about 3 months old. I got her from a pet store and she was already malnourished. She is now 2 years old and has become malnourished once again. I feed her cheap food just like my other two dogs. They both are healthy weight, but do have skin problems. I just changed their food yesterday to Benifal, Healthy Weight food. Will this help? I am so worried, because I can see her rib bones and her back bone. She has become slugish, and drinks an awful lot of water and her urine smells so terrible and has since I got her.