There are actually over 40 reasons that a good dose of vitamin D (10,000 IU daily) will not provide much boost the vitamin D getting to cells. Often just taking more vitamin D does NOT solve the problem. https://vitamindwiki.com/tiki-index.php?page_id=2296 gives all of the details. The page includes the reasons and the solutions in each case
Here are a few of the more frequently seen reasons.
Low Magnesium, Low Omega-3A, Anemia, Obese, Liver problems including fatty liver, taking a drug (such as a statin) which consumes vitamin D, taking a lot of Vitamin A, poor kidney, Vitamin D is not taken with/after a large meal,