Honestly the first thought I had was nitrogen toxicity. Are you cutting the suggested feeding instructions from FF in half or doing full feed?
my thoughts are "Manufacturers sell to make money. They hope that consumers will follow there heavy feeding schedule without ill effects to their grow. Use it up quickly and come back for more. I am not a fan of "hot soil" need to know what and when my plants are getting nutrients.
I would go with half strength.
I have used their products on veggies at 1/4 strength with good results. For MJ I use AN after failed tries with other name brand products. FF support will talk briefly about Marijuana, but seem intimidated / scared or just had no F@#ing idea.
GH and EH and GS would not even discuss Marijuana, polite but idiots. AN and I had a 50 minute conversation about their products in all forms of growing mediums for Marijuana (actually talked to a grower).
I look at it this way. " if I have to by an additional product to supplement a main product.........Than why would I buy that product?
without physical contact with your plant. All we can do is a best guess analysis. Wish you luck.