I seem to find that so long as your using a balanced and complete nutrient regime. The main issue is always over and under fert.
I try to keep my plants on a minimal p and k diet until as far as possible into flower (usually bumping it up beginning week 5 or so) because too much and it just locks out. If you have been feeding them that much and there hasnt been enough run through this could be the case.
I think cannabineers first edit is most likely correct. I would water through with plain water for a bit give them a proper soaking (lower the ec of the pots contents) and see if they start to look better you should notice fairly quickly they will just sharpen up a little bit.
I had a similar issue where I drove myself crazy trying to fix the def and in actual fact it was just build up and a good flush and keeping them fairly wet until the nutrients leached out helped them bounce back.
This is just what I would do and please bear in mind its just the stoned ramblings of a guy who likes to grow pot
Mouse