I'm not to familiar with the ingredients in the G.H. nutes. If you read the label sometimes they will list exactly whats in it. Check to see if it has Mg, Ca, and also if it lists any sort of sulphate like magnesium sulfate for instance. Chances are one of these elements is too low, if not then your concentration maybe too low. If you have a PPM meter then check to see your exact concentration. Should be in the say 700 - 1200 range if you get a 400 - 600 reading then sometimes this is it. Also if your solution is more then say 3 weeks old then just change your solution sometimes this will do it.

Herbus