Magnesium issue

[patpare]

I can't seem to get my magnesium level past 1200!
Everything else is fine
Alk:8.36 (Hanna test kit)
Calcium:430(Hanna test kit)
Mag:1200 (Nyos test kit)

I have been adding seachem advance mag on a weekly basis.
I do not use kalk as I read that it can affect the magnesium.
Any advice would be greatly appreciated.