Not eating enough fish

"Vitamin D" is actually a hormone produced by the kidneys and by the skin, which is an organ, upon exposure to sunlight. If your kidneys don't make enough of it, on their own, then your immune system could be compromised, (if you don't get much sun). Vitamin D3 and Vitamin D2 are the most important kinds.

Mushrooms are the only plant based source, if you like mushrooms. For your own safety, you don't want to eat fatty fish such as salmon more than twice a week because of the danger of mercury poisoning.

There are a large range of benefits provided by Vitamin D, and a deficiency could result in a weak immune system and in chronic depression.

Milk allergies and avoiding foods not rich people n vitamin d. Another reason to have a routine check up.

Also renal or liver failure

Dark skin, not eating enough foods rich in Vitamin D are contributing factors. My mom has both and her Vitamin D has always tested low.
Lastly having any absorption problems in your intestines such as Chrons disease.
My mom had gastric bypass which I think dosen't help because now they've given her prescription Vitamin D supplements.

Not enough taken in your diet