I have been using Oxypowder on and off for over 3 years. I am not sure what the quality of the multi-vitamin that you are taking. The multi-vitamin that I take is called Intramax which is a all organic liquid multi-vitamin. I am not sure if you have been tested for malabsorption which means that your body is not able to absorb vitamins and minerals. The only other thing that I think of that might be causing your problem is if you are taking any medications that might be depleting your body of the Vitamin D. One of the many medications that can deplete your body of Vitamin D is Actonel. This medication is prescribed to those with menopausal osteoporosis. The Oxypowder and other oxygen based intestinal cleansers will not cleanse the liver. You might consider doing a liver and gallbladder flush.
I hope this helps to answer your question.