hey dude i really cant tell you whats wrong, i once had the same problem too. Its most likely the soil, the nutes, and the distance of the light. My plants were dying as well when i used soil, but i decided to put them in a hydroponic system, and they sprung back to life. Incase you dont know, hydroponics dont use soil. They use a growing medium like perlite, gravel or rockwool (there are many more). They are put in a resivore of "working water" and nutes. You can make a cheap one for 10-30 dollors. You could use normal nutes with many trace elements or hydroponic nutes. If you want to try 1 more time then try hydroponics. Hydro can also improve you results by up to 75%. Make sure youre using the right nutes as well. High N, Low P and Med K for veg, Low N, High P and Med K for bloom. Your Light should be exactly 1 meter (over a yard) away from the tip of the plant.

Check this soil vs. Hydro test done by overgrow.com: http://www.overgrow.com/article/22
And check here to make a cheap bubbler bucket hydro system: http://www.overgrow.com/article/2
or look around online for one (nice full kits in ebay, just search "hydroponics").

Post your succuess