glycinate, citrate, carbonate, threonate, oxide etc etc. maybe another one will work better for you or perhaps it would be better in the form of juiced greens? then there is the liquid mineral supplement called Concentrace and it supposedly has all the minerals in ionic form, and 250 mg magnesium per serving, 96 servings per bottle.
or maybe some of your other minerals are out of balance and preventing proper absorption of magnesium? i think for instance there is an antagonism between copper and zinc, maybe there are other minerals that limit magnesium uptake.