Seed grown or clones from the same mother? From such limited info it sounds like toxic salts buildup which cause the older fan leaves to go thick then develop orange/brown patches, dry out and go crispy. This can progress into the water leaves then the bud leaves.
A good flush can halt the progress but affected leaves will not recover.
^^^The high N of the one with problems - Points to O,M,U's call as being on target!
I'm not a flush to repair, kinda guy but, this late. It's the best choice.....If it's synthetic nutrients...