Leaves bending down in center?

Like Mr weeds said, its a root issue either to much water or to much food. Since your in coco you should not be able to over water so I would say to much food. They look like they are stretching a little, I would either lower your light closer. 24"-26" at max power When your plants get older your going to be watering/feeding with runoff. You always want to have 1/3 of what you put in to run off( for example if you water 3L then you want 1L to of dripped out the bottom. So you will need to raise the pots so they do not sit in this runoff as that can cause issues. Another thing I don't see is your PH. If you don't have a PH pen or test strips then you will need to get some. PH is the most important thing to keep in range PH for coco is 5.5-6.5. Adjusting the PH is always the last thing you do before you feed/water. Happy Growing

Any form of downward curling usually indicates overwatering or overfeeding, and since your diary doesn't say you've given them any nutrients, this will be from overwatering.