So likely an N deficiency due to under-watering, the root systems are quite advanced and deep so the plants are most likely not getting the amount on N they need. I am currently hand feeding them when the lights come on with Phd water and superthrive. Ill add a 1 min feed to the cycle, so over an18 hours they get 2 x 1 min feeds and see how they go.
This is the Cocos from the bottom of the pot. I dug down and pulled some out and it's quite dry for the bottom so I would agree now more water is the answer.