What could be causing low vitamin d even with normal calcium levels?

Too little intake: The body maintains stores of vitamin d in the fat cells. It takes a long time for a deficiency to develop, and a long time to correct it using typical over-the-counter supplements. People normally get vitamin d from sun exposure and a few food sources (liver, eggs) as people have moved indoors and don't eat as much eggs & liver, vitamin d deficiency has become more common.

Usually poor diet: Nutritional causes are the commonest reason for vit d deficiency and then inadequate sunlight exposure. Only in severely low vit d levels can calcium levels drop.

Diet, malabsorption : Vitamin d deficiency rarely causes low calcium because if you can't absorb calcium from food (bec of low d), your body takes it out of the bones. Low d is usually due to lack of intake, but people with GI problems may have trouble absorbing it.