Mine have been scratching since August but they were growing their winter coats and now with the drastic change in the weather they're shedding their coats again. They had a medicated oatmeal and coal tar bath yesterday and their skin is clear of anything so I'm chalking it up to the weather. After their baths when I brushed them there was nothing left to come off. I know when we use the fireplace in winter they scratch more too and the groomer always uses a conditioner. A lot of dog owners are complaining of scratching this year so it must have to do with the weather.
Have you tried adding oil to their diets? Mine always get oil but they scratch anyway , was thinking of adding olive oil with the fish oil.
At least they've shed everything so they don't leave clumps of fur laying around.