Nitrogen toxicity? Or calmag deficiency.

Hey there... what WE need is a diary! Don't even know what you're growing or what nute line you're using. If you are indeed feeding every time you water, your plant has been overfed pretty badly and lots of that excess is staying in the soil creating a lockout of other vital nutrients. You need to practice a water/feed/water/feed system with the watering getting at least a 10-20% runoff. And I can well understand that the Northern Lights is the one most affected by this unless they are ALL autoflowers... autoflowers don't like as much nutes as photoperiods and even photoperiods are going to suffer if they don't get plain pH'd water inbetween feedings. You need to do a flush here... let's say you're growing in a 5 gallon pot... you need to run 15 gallons (3x the pot size) of pH'd water through each one and let them dry out. Since they are now in preflower, after they've dried out, switch to bloom nutes and only give them 1/2 strength for the first feed. I don't know how old they are but am going to guess that the ppm shouldn't be more than 250 for the first feed after the flush and 400 going forward. Good luck... please, start a diary!