Grow, Damn It! I Said GROW!

Well, hell. First time growing in rock wool, and I'm stumped. We have 10 white widow seedlings in 4-inch cubes (I know, too big, but it is what it is.) They sprouted 3 weeks ago, and they're all still less than 2 inches tall. They're leafing out nicely (2nd and even 3rd nodes on all of them), and seem all healthy and happy and everything, but they seem content to go through life as midgets.

I'm not growing bonsai here. I expect better of them. Is this typical for rock wool? I'm used to growing in soil, and would have expected them to be 6 inches tall by now, if not taller. We're feeding them quarter-strength HydraGro, and have them under a 400W MH bulb on an 18/6 cycle. This sound right, or should we up the nutes to half strength?

What's your Ph, ppm and feeding schedule? I'm fairly new..started my bubblegum clones under 400 MH @ 400ppm, next week to 600, then to 800, now at 5 weeks, growing and filling out [email protected] Flora 3 part seems foolproof...pH @5.7-6.2, temps 68-78..hope this helps..