plants dieing need help!!!

Ok before i was having a problem with my plants having yellow leaves at the bottom and they die off and the next set above that becomes yellow. so i gave them nutes. Fox Farm grow big 6-4-4 and i gave them half strength. And 3 days after i water them 2 of em that were looking the best have droopy leaves now, and the yellowing is still occuring, i thought they were underwatered so i gave em a little water yesterday but the show no progress. loos like they are drooping today more then yesterday and im scared i may lose them.. let me know what u think i should do.

And btw i have 2 ww plants that are droopy and 2 other plants im not sure what they are. and they are in fox farm ocean forest soil. they are 5 weeks and 1 day old. And the light is about 19 inches away/ 250watt mh. and it is about 88 degrees in the room at day and 70 at night. Also i had a pest problem before, they were fruit flies so i got neem oil and i have been spraying the neem oil mix on the top of soil every three days just to keep the bugs away and seems to be working good but i dont know if its that that could be killing the plants. And i dont have a ph tester yet but i got one in the mail on the way. its a rapitest 4 way tester.

The first and last pics are the ww and the middle 2 are unknown as of now. ty for ur time.
PLZ HELP

sounds like fungus gnats and not fruit flies, get some gnat off its a bacteria that kills the larvea, the neem oil keeps the adults away but not the larvea, neem oil good deterent. but not a cure.

ok so i got the stuff in the mail on the way now/// was wondering if i flush and transplant would i be doing good for the plant or doing more damage?

plants look a little under watered don't let them dryout.

that soil looks real dry get a moister meter some people weigh the bucket but thats not that accurate and u cant tell where the moisture is the top may be damp but not the mid or bottom they really help pick one up at hd under20

I have the same yellowing problem, and my plants are about as old as yours. I am still trying to diagnose mine. Is your yellowing happening pretty slowly? I have 4 plants and I tried 3 different things on them, so hopefully I will either find out if I have an N def, just needed flush, or cal/mag def. (k def). I will try to let you know if I find out. No one seems to know what is causing it.