Like GK said, temps may be playing a part, and def the nutes. Most nutes have about a 1 year shelf life and after that they under go changes that do things like what you are experiencing right now.
If your lights are not MH they will cause your lights to stretch durring the veg period and also, you want to get those 600w lights about 1FT away from the plants if heat permits. At 2FT away you can lose up to 20% of light penatration if not more. Closer than 12 inches you can lose out on your lights foot print and bleach the top coalas. I keep my 1K Hortilux about 10 to 12 inches away and with my 400W I have it about 6 to 8 inches away.
Good luck.
Adieu