I was (sadly) mistaken about arm training a couple years back. I "thought" that doing weighted pull-ups and weighted dips, would be enough stimulation to make them grow. Nada. Stayed at 17" for several years. Also developed different shoulder and elbow imbalances, possibly related.
This past year I have been more dedicated to direct arm training, sometimes twice a week. Now my arms are measuring just 18.5". Good pump and it's near 19".
Moral of the story - do direct arm training. Curls, pushdowns, extensions ... do it.