What ferts are you using? During the veg stage, you should be using something high in N(N-P-K). If you are not, that is most likely your problem.

What strength and how much fert are you giving them?

It's hard to tell if it's overfeeding or a N def.

Tell us what the n-p-k ratio of the ferts you're using, how much, how often. Knowing that we might be bale to help diagnose it better. If you are using no ferts, that's your problem.