There are a lot of reasons for reduction in Vitamin D Levels such as Aging, we automatically reduce Vitamin D production as we age.
Where you live, (above 35 degrees north latitude), and little or no Vitamin is produced from the sun from November to February.
Heath Conditions that create malabsorption and can lead to certain conditions such as celiac disease, crohns disease and hyperthyroidism.
Liver dysfunction and Kidney disease.
High smog levels in your own city, you can visit www.airnow.gov to find out your air quality in your area you live.
Medications, such as cholesterol lowering medications
Season ( no vitamin D produced in winter months)
Skin Pigmentation is produced 99 percent in darker color skin.
Sun screen use and sunscreen protective clothing SPF 15 reduces vitamin D synthesis by 99 percent.)
Now for things you can do to increase your vitamin D is through Vitamin C, Zinc, E, A, Probiotics, green tea extract, CoQ10, resveratrol extract, Sea Buckthorn.
If you sit outside for 20-30 minutes with out sunscreen on arms and legs,can help increase your Vitamin D levels, but skill recommended for your face, hat and sunglasses.