vit-D puzzle?

[VDC]

What are the current thoughts on Vit-D? I just requested a plethera of labs and some have already been processed (just had blood drawn today) What surprises me is my vit-D is low again at 19.3 ng/mL AND I have been taking 5000 IU's of vit-D every day for almost two years! last year my vit-D on this same supplementation was 67 ng/mL and now with no changes in consumption, it has dropped to 19.3 I am very very puzzled. Does anyone have any idea what could cause such a dramatic drop in vit D even while on significant supplementation? Are there any things that I might be doing that can cause vit-D to be reduced? Very very puzzled......