It looks like a deficiency of some sort. First of all, for soil growth (and any medium with 20% or more soil in it) a PH between 5.8-6.2 is best and up to 6.5 is ok. Out of this range, nutrients uptake starts getting locked-up by TDS. If I were you I'd flush them with a flushing product (the same you use before harvest). You have to water them until they drain at least 20% of the water you gave them. So let's say you mix the water with the flush product as suggested in the label. You prepare an 8 liter (2 gals) tank, place one plant in a basin to collect the drained water. You have to drain a minimum 20% of 8 liters that is 1,6 liters (make it 2lt. or half gal). Use an EC meter to measure TDS in the drained water. Leave the plant to dry out for 24 hours, then prepare a fertilizers mix as per schedule using the minimum dosage indicated in the bottles and feed them. You can feed them once a week and water only (PH adjusted of 5.8-6.2) the rest of the week as needed. Repeat for each plant.
Point is that you need to drain at least 20-25% of the water you give them and drain them once a week to avoid solids accumulation which will prevent your plant's ability to feed itself. Too much food blocks the food uptake and makes the plant show deficiency. Feeding more won't resolve as food is there, is just stuck. Flushing thoroughly should do the trick. Later on, while draining your plants keep and measure PH and EC (TDS) of the drained water so you know if you're "in the zone" or not.
After flushing give the plants a full day dry (24hrs). The following day you can start over the feeding. If you follow a producer chart or the indication on the bottles, remember that the amount of fertilizer shown in the charts is not for "each and every" time you water them. Start giving them the "light feeding" schedule, let's say the chart says light=20ml normal=30ml Heavy feeding 40ml. You wanna start with 20ml (in this example) but only once a week, the other days give water only (adjusting only the PH). If you wanna feed more than once a week, you can feed the way you like but the same total amount. If you start with 20ml and want to feed twice a week you give 'em 10ml+10ml, 3 times a week is 7+7+6 and so on.
You also said they're on a 12/12 light cycle?! Did you put them in flowering? I don't know if it's reversible and anyway it'd make you lose a ton of time anyway...if I were you I'd keep 'em in veg state (18/6 of light) until I solve any issue and they're healthy, but if they're Auto's or already flowering I don't know really, seek more advice on this.
I strongly advise you to keep a growing journal. There are apps, online journals, homemade feeding schedule, it's your choice. Record everything, from impressions to values (Temp, RH, PH, EC,...) to feeding. The more you can record, the most you'll gain from this operation in terms of learning more than in crop+.
Participate in forums and buy a book. Yeah, a book, in the 21st century, yeah! You'll be surprised how different and easier it becomes studying on a book, where you can go back not once, but 2,3,99 times. You can put reminders as well as highlight the parts you know you're gonna need in a later stage. Try to learn the most of a stage before it begins, thereof you'll have the proper knowledge to deal with errors and horrors!
Good luck to you 'n the babies!