I have a 10 month old GSD, I usually give her some tripe with her dry food, it's really stinky but all my dogs like it, they also like Butchers tripe (tins) don't give too much though as she is on a complete food, try also a tin of sardines or tuna, dogs love stinky things, try her on a different dry food, try Beta puppy/junior or Bakers puppy, add some veg and a bit of cooked pasta or brown rice I give a bit of pasta to mine (I have 3 GSD's and a Border Collie) with veg, cabbage/peas/carrot/swede/cauliflower you can give broccoli but not too much and no onion as it's toxic, you have to jazz her food up a bit and make more interesting, the tripe in particular is good as it will help to put weight on her. Give it no longer than about a week then take her to the vet again. Don't feed her ad-lib, if she refuses a meal after say 10 mins take it up and don't give her anything until the next regular feed time in the day, but as I have said please don't let this go on for more than a week, if you are not happy with what your vet says then go see another one.
Good luck and please let us know how she gets on.
PS. when you give meat of any kind mix well in don't just leave it sitting on the surface I find this works well. and by the way I'm not suggesting that you try all these things at once but I alternate things to keep things interesting.