This is odd… I’m inexperienced too so I cant give too much input, but I will say that Jacks321 is meant to be used each watering. It COULD be the cause of this if one plant is growing faster than the other due to genetics, therefore requiring more nutrients… Getting a dude like @Hellraiser in here on the issue would be of much better help. I know with his Coco grow he worked his way up to full strength I believe. At this stage in your plants life, I would say you probably need the full strength each watering…
Also, you are obviously in flowering, not sure how far you have yet either, but most growers do a flush and start giving straight water for the last few feedings. This is supposed to help get out the nutrients and result in a smoother smoke.