Still not getting anywhere with my vitamin D level.

I am now in my third year of taking 50,000 units of Vitamin D. I take it once a week for 8 weeks then once a month. I also take 1,200 IU OTC vitamin D everyday. My level started out at 8. It reaches a level of 18-20, then drops down to 8 or 9. This has been going on forever. What else can i do? I fear i will never reach the correct level. I incorporate a lot of vitamin D enrich foods in my daily foods, such as salmon. I also have fibromyalgia. Why does my level keep dropping. My doctors do not know. Thank you.