Welcome to RUI!! Sounds like a nitrogen deficiency or you may be watering them too much or too often or not getting enough light. If you are growing in soil, you should be watering them every 3-4 days really good and letting them get nice and dry in between times. Light is extremely important; that is where they get their energy from.
i think it might be the nitrogen deficiency and watering it too much.
i think the light i have is good until i switch to flowering.
but thanx for the advice MOBee and potpimp.