I use the fox farm line, and I usually encounter the same thing about 3-4 weeks into veg, thats with only using the grow big, then I add a little bit of big bloom to the feeding schedule as well and it usually clears up the problem. Its possible that its a Magnesium deficiency. Thats what I originally thought with my similar issue, and I bought epsom salt to clear it up, but the big bloom did the trick so I never used it.