Quick question - did you feed veg nutes throughout veg? If so, how much, how soon did you start feeding and how long did you veg? If you fed veg nutes, you may have over-fed, creating salt buildup in your soil and locking out everything else - even with those light amounts, as FFOF is pretty heavily amended. As for your original question, a little CalMag is nice.
Not that it helps to answer the question but I use to use fox farm and had problems like this among a horrid of other problems. I finally switched to General Hydroponics and ill never go back to Fox farm the GH has been stable and predictable throughout 2 grows now with no plant problems or ph problems.