I believe they need to be jacked up with mag when they go from veg to flower under the brighter lights. It's basically like light sickness. Too much light, upped co2 levels and not enough Mg to facilitate photosynthesis. Last grow I had no problems and they stayed good and green. This time I just forgot, which is why it's good to take notes and then read those notes.
When I say jacked up I mean adding like 10 ml's per gallon of calmg for the week before they go into flower. That for me seems to ease the transition.
Also applies to my True cut as well.
that's interesting, when i ran my first OG in a long time i got the 'light sickness' when i transplanted to a flower room. Big Ricky suggested epsom foliar and boom next day they were lush green and perky.
10ml calmag+ would be about 1 EC... seems like a lot? do you drop your base a lot?