I'm having an issue with some strains yellowing too early in flower. Gg4, gelato, etc... Is it the N ?
are you in soil? If so feed heavier going into bloom and through first couple waterings with v+b then back off. Example i use to do would be 800ppm going into bloom through 21 days of bloom then back off to 600ppm.
Another option which im doing now is run Megacrop in veg, its better then V+B in veg. Then switch over to V+B on day 1 of bloom.