just out of curiosity Doc, are they all the same variety?
or is this one a different strain?
looks like you are watering/feeding by hand (cant see any tubes or evidence of flood and drain, or any pot saucers)
she is located in the middle and maybe not getting the same share as the others.
just spit-balling here mate;
someone elses perspective can often throw light on a problem that isn't obvious to the person closest to the problem;
"cant see the wood, for the trees"...that sort of thing.
the older growth is fading that's obvious; and I have definitely seen that in a list of deficiencies.
potting mix looks high in perlite and very dry.
If theres a chance that salts might be accumulating to a toxic level,
try flushing and then perhaps a tonic, nettle feed, or molasses. segregate if you can.
start with Brimans suggestion and cross-reference the symptoms with some online research.
will be interested in the outcome.
good luck!