There are several possible reasons. I have had some clones that just start out that way then during veg correct themselves. It could also be over watering or root over saturation. I assume you have perlite or vermiculite mixed in to help air and drain. Without knowing the ph of the water going in and what the run off is it will be hard to pin point it exactly from a far.
Edit.... I just looked again it looks like your in cups with that heavy bark type medium stuff so I am going to say over saturated as I cant tell but do you have holes punched in the bottom of the cups so watre can run out and not sit and pool as that can cause over watering and root rot so bad O2 intake of the roots which means nutrient build up,lockout, and sufficating the plants roots.Not a good medium and those damn peat pucks always cause issues and run acidic.