Hi there
@Lungren 
Nice grow so far. You have yourself a case of Calcium def by the looks of it. Don't get too down about it, all us coco growers have seen it
Might I ask which of the promix soils you're using? They're not all the same. Nor I believe, do any of them contain coco. Mostly a varying mix of Peat Moss, Humus, Gypsum, Limestone, Mycorrhizae and presumably some form of fert although it's not listed. Seems like good stuff to be honest.
50/50 mixes like this make it a little more difficult in my opinion but I would listen to your plants regarding feed frequency, If they're light, feed em, if they're heavy, don't! There's no set rules and everyone's environment is different.
First question is how much cal-mag are you giving them? You should probably be looking to about 2ml per litre if it were straight coco but that can probably come down a touch given the presence of Gypsum (basically just organically sourced calcium) in the mix, assuming there's enough in there.
I might also suggest that your PH should increase a bit. You're bang on the money for coco, but too low for soil. If they PH is too low, it won't be able to do anything with the calcium regardless of how much you feed her. Anything below 6.0 is lockout land and it won't be able to take in phosphorus, calcium or magnesium.
Whilst we're at it, you might want to check the PPMs if you can. If you've not got a pen, get one! They're cheap. If they're too high you can also get a lock out, but PH is the most likely culprit for lock out given what you've said.
As others have mentioned, with the mix you've gone for, you should probably treat her more like coco and be giving low feeds every time, she doesn't need water as the mix isn't going to be rich enough to give much food to the plant. Go low though and just keep increasing it until you see a little burn then back off again.
It's relatively easily fixed and you've caught it in time. If it makes you feel any better I've just lost a plant to similar troubles.
Happy growing!
