...yeah, they are definitely old enough to need feeding, lol.
...like i said in the other thread though, i'm no soil expert, my guess though would be maybe a ph issue and also maybe some over watering.
...i'm sorry i can't be more help but i grow in coco hempy buckets which is a passive form of hydroponics that is much simpler to run, at least for this old stoner, plus, i feed KISS style using only Jack's Pro Hydro and calcium nitrate so issues like yours just never come up, ever.
peace, bozo