also sorry blwoig up this page loll but you say run off u can test the water run off from cups to get accurate ph mesure or is it better to use soil tester itself?
My bad, I wasn't thinking your girls are in cups. Good thing you mentioned FFOF, because that's what I've been using. I just found out that FFOF gets more acidic as it gets older, which I didn't know till after I destroyed my first grow. It's possible you picked up an older bag. I ask now when new shipments are in, and then try to grab one on the bottom or the back of the lot.
The best thing you can do is flush, but they're so young and in cups there's nothing to flush. Maybe see if you can get some powdered dolomite lime, mix it in with the FFOF you got, and replant them in larger pots.
I'm not saying your FFOF is too acidic, but that's pretty much the way my last grow went. You'll get a nice harvest, just not the harvest you want.
FYI, this time around I couldn't be asking for anything better, and the FF liquid nutes are working too. I'm growing indoors. Let me know how it turns out.