D3 has a very short half life. If a person is taking it and then stops for a while and is then tested, it may not read accurately.

Also D3 levels vary from person to person depending on how much fat they have on their body. Heavier people, may test low, as the D3 is stored in fat to some extent.

I do believe it takes TIME to even out levels.

Studies show that D3 in dry form and oil form both are effective.
There is such a tiny amount in a capsule or tablet... it should not
be causing side effects at all. The dosage number may look like a large number but in effect only tiny microgram amounts are in it.

The large RX D2 is a big capsule...it also has sorbitol in it.
But the tiny OTC D3's, have a drop or so of soybean oil instead.
The powdered form for some reason in some OTC products seems unnecessarily large IMO. I take the tiny caps from Puritan's, with no problems.