Hello, it looks to me like you have some phosphorus deficiency it looks very similar to calcium deficiency the way to tell them apart is P will show on older growth and C shows on new growth. P deficiency is normally caused by watering issues or low temperatures. It sounds to me like you are feeding them right leaving me to think it is over watering but without knowing your habits I can't be sure
Hope this points you in a good direction and you can figure it out... good luck đ
Cyrus