Plant Problem

I'm growing in a hydro system under a 400 watt MH just over twelve inches from my plants. The system has an 8 gallon reservoir into which i've put 6 tsps of Dyna Gro (the recommended amount) and the pH is right where it needs to be. Three of my plants are doing very well (although two were stunted, long story) but one is droopy. I topped the two big ones last saturday (2/10) and the other one is doing just fine under the exact same conditions. Any suggestions because I am clueless? Pics attached.