Kind of sounds like a PH issue but without pics that is just me assuming, for me when I have problems I go back and check all my basic stuff first. Make sure of your PPM's and your PH because that info is vital to diagnosis, also your climate can tell you a lot as well such as temp minimums and maximums along with humidity. You also want to know what the temp of your res is running at as well which plays an important role in PH.
Good luck and happy growing.
KnB