Gloria-
Our hen Penelope had something like "dandruff". Very flaky skin and/or feather shafts (you could see larger pieces of what appeared to be broken cylindrical flakes off the feather shafts). She had this when she had an impacted crop which was affecting her nutrient/food absorbtion. She also got very skinny. I don't know for sure, but I felt it was due to the lack of nutrition.... like when people are very ill and their hair and skin get dull, flaky, hair falls out..... anyway when she recovered and started gaining weight again, the "dandruff" improved.
Is your chicken acting fine otherwise? Not losing weight?
Maybe you are right about the dust bathing, though. This hen of ours had just come to us a week before and was still getting used to her surroundings. maybe she was too freaked out to dustbathe..... and then we brought her inside to a dog crate while she was sick..... I guess it's possible it was lack of dustbathing!
Why not set up a dust bath for her and then see if she still has it?
Stacey