Can someone help me with my dog ?

Your vet said there was nothing wrong? I think the vet could notice it right away. Call the vet again and see what they say. A good food would be my opinion Pedigree try using that see what happens. Or maybe use soft food instead of solid. Keep on shampooing her see if it helps. Also if she has dandruff you should brush her with the right comb. If she is short haired uses a short hair brush. (I've never seen a long haired great dane put if it is which is most unlikely uses a long haired brush)