5,5 is a bit too low for coco but still your plants look fine. Whenever i have a problem with nutes foliar feeding always helps. Mix a half strenght solution of what you are feeding now and spray it all over the plants(do this when lights are out) If it is a nutrient problem this will fix it and the plant will love the shower.
I would argue that 5.5 is just dandy for coco, and that 30ml of nutrient per 5L is too much given the age of the sprouts.
Or, there is something besides coco in the coco and it wasnt' mixed in well enough, but I didnt' see anything about amendments to the coco in the infirmary thread.