Need Help: Plant Wilting and dying

i have a bucket system. I am at week 3 bloom and 3 days ago 1 plant wilted and pretty much died. today i came back and another 1 is wilting and dying. I started over a year ago and had no problems with my first crop. It was in winter though. It is summer now, but i did install an ac. the room temp high is only around 85 and that is right below a light. after my first crop this happened for the next 3-4 crops. between every cycle of plants i bleach the whole system out for a week then run fresh water through it for 1-2 weeks. Im thinking they are getting over watered but want to confirm. i am watering every 4 hours for 15 minutes. i just set the controller to water every 6 hours now and am hoping this will help. i dont want to lose anymore. both the plants that did this are not the biggest and it seems that the root structure never got that big. this may be a part of the problem too. Im really not sure and am looking for help. i have attached a picture of 1 of them so maybe that will help. thanks in advance for your time and help. its much appreciated.

looks to me like your not getting the air the roots need did you get new air stones for this batch?

Did you change to a bigger dose of nutrients recently it lookes like it got too much. But what kind of bucket system are you using are the individual buckets that hold there own solution or are all the buckets connected to the same reservoir. If its a dwc bucket set up it could be due to not enough oxygen in solution but only if the roots are suspended down into the solution it self.

i have an air pump with 4 outlets on it. there is alot of oxygen being produced in the res. i typically lower my nutes for the first 1-3 weeks of bloom. my last weeks of veg are high in nutes. i am using the green tree bucket system with does go back to the res. i watch the ph which i keep at around 5.9-6.1 and the ppm's are around 1500, which is low compared to what i normally do. the 2 plants that died did not have a good root structure. one had no roots out the top bucket and the other only had 2 roots coming out. i also have a couple plants that are maybe 12 inches, that never did good in veg, but those arent even wilting. its wierd.

Any ideas. I lowered the watering from 6 a day to 4 a day. And I just had another one die.

what is your medium? strictly hydroton?

Yes. I'm doing everything I was doing before. I just don't understand.

anyone have any ideas. after changing the watering schedule will some of my plants still die. were they bad from before and not recoverable. i cant seem to find any info on this stuff. i just dont understand the small ones are not dying and the big ones are dying from overwatering. please help