Internal Parasites..........worms etc
Diet...grain filled food
Ear Problems..........from mites to yeast infections
Care........lack of grooming .....Shedding their coat can cause itchiness........too much bathing dries the coat and skin, not rinsing out all the chemicals from the shampoo will also cause this.
Anal Gland Problems...often due to diet/ be over weight
Environmental..... using chemicals on floors, carpets, or lawn be aware that these chemicals as it can make your dog itchy. Even the detergent used to wash a dogs bedding can make him, or her, itchy.
Climate....Dry climates often result in drier skin, and therefor more itching. Dogs can suffer from sunburn which, as in humans, will cause itching.
Mange.....caused by mites
Fleas.....cause itching and worms
All these can be helped with dily grooming, a good grain free food or better still Raw and Bone, using neem oil externally and powdered neem internally which boosts the immune system, it also protects form parasites ( internal & external) and promotes a health coat/skin/teeth/bones. Be careful of putting neem oil on or near open sores as it is very powerful and can cause more issues...so these need salt water bathing.................without seeing your dog it is impossible to tell however if it is not fleas it is likely to be mites/mange.....a good groom, a bath with an oatmeal dog shampoo with some neem oil added will easy it, then make a neem oil spray and groom daily ( plus a grain free food)...however if this continues then go and get it checked out professionally
http://www.discoverneem.com/neem-oil-for-dogs.html