You need to be pH-ing everything you give your plants... the plain water pH'd to 6.2 (ideal) and the nute solution also pH'd to 6.2... When you water, water enough so you've got at least 10-20% runoff and test the pH of that runoff... water it until that runoff is between 6.0 and 6.5 - this will help you understand what's going on vis-a-vis the pH inside the soil AND it will help clear away and built up salts or excess nutes in the soil and help prevent a lockout from occuring...
General Hydroponics PH-Down is the best on the market for tackling the problem....
Good luck!