Good suggestions. You can have seb derm that doesn’t look obvious.
The Derm scope would pick up on scales.
Patch testing for chemicals affecting the skin is usually done by dermatologists if they have an interest in allergies. They may not think
of it if there is no outward signs of irritation but the problem can exist
without being apparent.
The internal causes may require some blood testing to eliminate deficiencies
or metabolic problems.
There are other treatments you can try. I have subtle scalp / ear psoriasis and
and do well rotating several shampoos with different active ingredients.
Ketoconazole works best if massaged in with a baby hair brush and lather
left on during shower.