Great lookin' "Lucy"!!,......Ive always found that 40% or above ambient humidity generally does the trick. Also a large water bowl will help raise it some as well.
That seems really weird as healthy as it looks, that it hasn't shed in that long a time!.
I would put her in a container of tepid(slightly warm water), or a pillowcase full of very moist spagnum(or similar) for several hours(even over night)just to see what happens. It certainly can't hurt, and would PROVE if their IS an old layer or two of skin.
As you well know, that's pretty uncommon, unless you're home heater is really "pourin' the coals" to the house.
In any case, see what happens, you might see a shed in the morning!............best regards,...........Doug
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Better to be silent and thought a fool, than to open mouth and remove any doubt!