Did not take enough

Some medical problems need more than 5,000 IU to get any results

Some peoples, like Eskimos/Inuit, have intestinal systems evolved to extract less vitamin D ClICK HERE for Hypothesis

People who are obese get approximately 1/2 the blood level of increase for a given dose of vitamin D

People who are very deficient need to first replenish their vitamin D levels, sometimes with a high single or short term dose, before going on a maintenance dose

Very deficient include seniors, submariners, women wearing burkas, people avoiding the sun !!CLICK HERE

Had taken vitamin D2 instead of vitamin D3

Doctors in the US can only prescribe D2, which lasts about 1/2 as long in the blood stream and in many cases seems to be 1/3 as useful

CLICK HERE

Had been consuming something else which had decreased level of vitamin D in the blood

Vitamin A, Statins, Chemotherapy, smoking, anti-epileptic meds, etc CLICK HERE or  HERE

Medical complications

Malabsorption (short bowel, pancreatitis, IBD, celiac sprue, bariatric surgery,..)

Liver disease or failure (liver converts supplement into what is used in the blood)

In hospital - 57% of seniors are deficient

Poor vitamin D availability

Some pills have as little as 10%. oil pills are consistently good

Vitamin D reduced so low that Victorian age diseases are returning