What kind have you been using? Not necessarily the brand, but is it a chelate?
Angstrom minerals have a particle size small enough to enter the blood stream directly, thus bypassing digestion and the digestion ills that mag can produce. This stuff is costly, but magical. Hubby and I take a TBL twice a day (you have to put it in juice or something - it tastes awful) I personally use a swig of chocolate coconut milk.