Help with a plants in my first hydro grow

[torchedlh]

This is my first hydro grow. it seems that most of the plants look extremely healthy except for a few. this is the rundown of my ebb and flow system:
40 gallon res, 15min flood every 4 hours.

GH flora series nutrients, at around 350-400 ppm.

I have not changed the water for about 10 days, but plan to do it tomorrow. I have been watching the ppm everyday and it seems to drop a tiny bit but i haven't added been adding nutes until just yesterday, i brought the ppm up to about 450.

i adjust ph to 5.6 daily (it seems to rise about .3 each day). note: i am using cheap meters, a hanna ph and HM digital TDS meter (i will uprade these next grow).

The lights i have is a 400w hps and 400w mh in that one dual reflector. it seems as the ones in the back (who get most mh light) seem to be doing noticebaly better than all the other ones. the light is approx 17" from the tallest plant (center right).

temp is about 75 with lights on and about 68 when the lights are off. RH is approx 40-50%.

i also have a co2 system regulated, .2cfh for 1 hour a day, then 12 15min cycles during day.

all help is appreciated. thanks

[torchedlh]

anybody??