Hi, did you consult your vet?
When I had a weight issue with my pup, it turned out that he lacks vitamins and that's not something you want to hear when you think you are doing it all right My dog wasn't that much skinny, he was more lethargic and when I went for consultations, it turned out that lethargy can be fixed with better nutrition and supplements. I felt relieved when vet told me that it's not so serious and that she has no any illness... But she needed more balanced diet, more protein, more vitamins etc.. I switched to natural Ivorycoat dog food and I am still satisfied with it! I believe there are different options and you should find what works the best in your puppy's case. But, you should first go to vet, you need to make sure if her 'skinny' condition can be fixed with nutrition... You should ask him for advice, but out of my experience I recommend to stick to natural options, with various protein sources... I hope your pup will get better (and 'fatter')