What nutes are you using? I know you probably said but I am lazy...doing school work and reading forums at same time. Do you know if the ferts you are using have micro nutrients?....If no, you can switch ferts or use an epsom salt mixture with your current ferts, which will get magnesium to your plants. Cal/mag can be used, but I know a lot of people use epsom salt just fine, and I believe it's pretty cheap at Wal-Mart. Use about 1 gram per gallon, and if you foliar spray it, you can get it to your plants right away. Do you know if the ferts you are using have any salts in them? It could be that you have a build-up in your soil that's causing magnesium to be locked out if your ferts do have micro nutrients in them. If that's the case, you need to flush, adjust your Ph to be mag friendly (have to jack it up for about a week) and then feed a supplement (epsom or cal/mag).