Experienced grower with first time problem help needed

I am running a dwc bucket system with a central reservoir. Everything receives the same nutrients, same room, same lights , only difference is the strain the three that look good are Pineapple Express the two struggling r LSD. What is the problem and what is causing it ?

im going to take a quick guess at this but i believe that certain strains do better at certain levels pertaining to Ph and your nutrients ppm and .. i think i saw a grower on you tube having the same problems but with a soil grow, look into your genetics and you'll be sure to find info on the plants maintenance.....Keep On Growing

Either the ph is out get a calibrated meter and make sure res is 5.8 or the temps are hot now or it was hot and that strain is sensitive to heat.

Two brand new constant ph readers used and ph does not go above 6 ever Temps are perfect both in the reservoir due to the chiller I use to ensure 68 degree water and temps have never risen above 77 or below 58 because they to are constantly monitored. Look at the other three plants all receiving the same everything as the effected one I am not discrediting what u r saying I know they would cause that but I also know that it isn't the case with my room I have been doing this over 10 years and have spent over 25k improving my room this is the first time I have had one plant out of the bunch react like this

some strains just dont work together. i have 14 strains that do great together but it took me close to 45 to get those.

Are all the plants of just one strain showing it?

There are two of different strains one is doing ok nowhere near the other three of a different strain which are doing awesome and then there is one that just looks like crap I have decided to cut it down and get rid of it so the others have more room