Response in the blood to a vitamin D supplement can be much lower for some individuals Reasons include:

• Vitamin D not taken with a meal which had mono-saturated fat (such as almonds)
• Person did not have enough Calcium or Magnesium
• Person was elderly, overweight, had gut problems, was smoking, had different genes
• Person was taking drugs which block or consume vitamin D

CLICK HERE for details, more reasons, as well as a great chart