what could this be?

Frequency of watering is likely they cause. If trying to do multiple fertigations per day wait for the plant to be large enough to drink down at least 25-33% of mass from a recently watered pot. Doing this sort of technique too early can possibly hinder root development. There may be benefit to allowing roots to develop more robustly with a minor amount of stress from a more exaggerated wet-dry cycle in early vege and into vege. then, once they have colonized pot and drink fast enough, you can ramp up fertigation frequently later and match growth rate of any hydro context, all other factors the same. Also, if pot size and plant size don't jive, better to stick to a safer wet-dry cycle. People succesfully do that method as well as constant recirculation systems (or at least very frequently cycling). All about maintining 'on demand' levels of nutes that match growth rate and also keeping enough o2 in rootzone. Watering all the time can work, but requires a specific context to be successful. doesn't have to be coco. 50/50 perlite + spagnum peat moss will have better drainage qualities, more air penetration and hold the same volume of water per same pot size. It's about cause and effect and not memorized magic.