
Sounds like it might be sarcoptic mange or cheyletiella mite also known as 'walking dandruff'. Mange isn't always picked up in a skin scraping. Here's a link to some more info on both
http://www.marvistavet.com/html/sarcoptic_mange.html.http://www.provet.co.uk/health/diseases/cheyletiella.htmDepending on the breed Stronghold is used to treat both of these, so I would ask your vet about trying that. (It's toxic to certain breeds such as Collies.) We had a problem with itchy dogs and spent a fortune on frontline and various other flea treatments, not to mention consultation fees, but a course of Stronghold sorted it in the end. The vet didn't really identify what the problem was so they just tend to go for the most likely thing and said it was probably a flea allergy or something in the environment.