How far north do you live and how dark is your skin? If you live in the higher lititudes, you may never get enough UV light to make Vitamin D even you are out all day.
It’s been a long time since I sat in a biochemistry or a physiology class but I do remember that there is a maximum dose of Vitamin D that your body will make regardless of your skin color or UV content of the light. The body modulates the ammount of precursors available in the skin. Your thermostat may be set low.
Strange that you run low, you are definately a good candidate for supplementation. What do you run with 2000IU/day?
I am in Texas where there is plenty of sun, and have fair skin. With the supplement, I stay in the mid-low ranges of normal, but high enough that more D3 is not needed...