Odd vitamin deficiency

Patient: Hello,A few months ago I went for a blood test, and it showed that my vitamin D count was 14.5Since then I was put on a dose of 1600 IU a day. It’s been five months and my count has only gone up to 85, even though I’ve been taking them religiously every day.What could be the cause of this?Thank you in advance!

Symptoms: Vitamin D deficiency