It’s possible you’re not getting enough precursors to allow for sunlight to greatly increase vitamin D production. How much fat do you eat in a day? Skin tone is an additional factor as well.

Also, an aside, it’s humorous to me that you describe 30+ minutes a day as “plenty of sun”. By todays standards, definitely. But the natural state of humankind is “however many minutes there is sun” worth of sun per day.