Do not let the coir dry out even the slightest. The roots should be racing through the pots.
How often do you feed? Do you use water at 20 c or room temp? Do you water until you get run off?
They look a bit hungry? It is worth experimenting on a few with an increased feed maybe.
I am watering every other day about 700ml of water (11L pot)and water always run-off. Now I give 3-4ml/1L PM coco A + B and 5ml/1L PM root stimulator. I noticed that after increasing ferts plants began grow better. I'm not sure the reason of slow growth could be poor nutrition? Temps of water is 18C-21C. EC1.40,PH5.9. Thanx for replys!