Apples and pears contain sorbitol which give them nonfermentable
gravity. The effect you describe does occasionally occur with some
varietys of pears. Apples the effect is always present but is
ususually quite low.
Stuck ferments on apple is sometimes due to zinc deficiency Years ago
I used to add twenty grams of zinc sulphate and five grams of thiamine
to twenty five thousand litres of ferment this was enough in most
cases to unstick a stuck ferment.

Bob M
www.molab.co.nz