Not hitting target serum concentration despite 50.000 IU/d loading dose & cofactors

Hi, I've been trying the Vitamin D protocol (https://vitamindregimen.com) for Cluster headaches, with a loading dose of 50.000 IU for 16 days at the start of january and a maintenance dose of 10.000 IU since, to try and hit a serum concentration of at least 90 ng/ml (purposefully high because of my neurological condition, you can read more on that website link). All of that approved by my doctors.

However, more than a month since I started that, I still have only reached a serum concentration of 50 ng/ml (up from 20). What could be the reason for this?

I believe I have all the necessary cofactors too?

- [ ] Omega 3 Fish Oil 1000 mg/day

- [ ] Magnesium 800 mg/day (magnesium glycinate)

- [ ] 1 Vitamin K2 MK-7 200 mcg/day

- [ ] Vitamin A (Retinol) 700 mcg (2,333 IU)

- [ ] B1, B2, B3, B5, B6, B12, 200 μg vitamin B9 and 150 μg vitamin B7

- [ ] Zinc 10 mg/day

- [ ] 1 Boron 3 mg/day