Not enough water, you need to water the entire pot until run off, not just little bits of water right next to the stems, the roots grow sideways, not just down. Water the pots from edge to edge and until it runs out the bottom of the pot.
Also, those pots are too small for more than another 1-2 weeks of growth and you need to go to 18/6 - 12/12 from seed is rubbish, the plants will be in a constant state of struggling to make enough energy to grow properly.
Hope this helps, Organoman.