Frequency has less  to do with it than quantity, if you over feed once the damage is started. There seem to be several reasons  one plant out of 3 or 100 will look bad, its the same reason I am handsome and my brother is not I suppose or maybe FF didnt mix it well enough that day. What is the PH your running at, maybe the others had a little nute lock so they held back a little. Another good argument for hydro and clones, theres no dark corners for genes or nutrients to hide.