put up a picture it will help to properly diagnose.
an more info is needed
temps night/day
hours of light
humidity
wattage of light and type of light
distance of light from seedlings
have you given any fertilizer yet
what brand soil
how often do you water and whats the temperature
whats the pH going in and coming out
These are things that help were going to ask you and they all make a difference in the way a plant grows. However being site unseen I will presume that you have been doing it right and not feeding seedlings. So I think its time to feed, and the plants nitrogen deficient.
Again, gonna need more from you