I have chronic problems with Magnesium assimilation, also. I sometimes get those popping sensations just under the skin, and also very tight, crampy muscles.
Having an overgrowth of candida somehow interferes with cell receptor sites. The cells just don't get proper electronic signals letting the perimeter of a cell know that a certain substance is waiting to enter.
Another way I get Mg into me is Epsom Salts which is high in Magnesium. Follow the instructions on a bag of Epsom Salts , I think it's 2 cups per bath of water, but check the label. Dissolve and sit in it for 20 min then rinse off in the shower. I have also used Epsom Salts as a foot bath. In this way I'm absorbing Mg through the skin, without it having to go through the digestive tract, causing diarrhea.