my 2 year old german shepherd did the same thing since i brought him home for the first time, it would get worse in the winter like real bad he would scratch so much hair would be missing around his hips i brought him to the vet i got nothing out of it just wasted 300 bucks that day i think it was his diet i stopped giving him milk now hes good.
Assuming you have checked for and discounted mites (as Mark has said) ... Dogs also scratch when they are moulting, because the loose hair makes them itchy. Regular daily brushing will help alleviate this. If it is dry skin, ask your vet for a good moisturising shampoo and follow the instructions (cheaper versions are also available at good pet stores). It might also be something you are using indoors that your dog is allergic to. This is much more difficult to identify, but obvious red-flags are sprays that you may have recently started using (particularly any used on carpets or floors or those sprayed into the air that will land on floor coverings). Tony
Could be dry skin or mites, like ear mites in your dogs ears. Cats get these mites too. Check the dogs ears and if not, maybe could be simple
case of dry skin or possible allergy. Check with your vet of course.
Mark