Man i had the same problem before i got my grow room vented.. Mine was due to heat stress. The temps where getting up in the 90'sF.
Yours could be something minor, or a combinatoin of several thing.
Like Pool said we need more info.
Soil or Hydro?
Lighting, how far away?
Temperatures, highs and lows?
How old are they including veg time?
What nutes do you use?