Help needed /hydro grow

Ok just did a res change yesterday and flushed system and plant roots to see if it would help and it was due , Roots are white no sign of any issues ,its been a strugle learning to read the plant but I am getting it, So res change start 387 ppm today check 384 ppm PH 5.9 PH down to 5.7 everything has been good untill 3 day ago the lower section of one of the plant has leaf and stem drop (drooping) mostly new growth,but the upper half of the plant seem to be doing fine, sorry about the lighting on the lower pictures of the plant ,could not block the 400W MH and take the pictures at same time,temps and airflow are stable 68 lite off and 78 lite ,Please need advice here on whats going on

need more info ... undercurrent ? strain ? how far along ? ppm seems low I veg in flood tables between 450-550ppm

strain THC Bomb , just under 6 weeks from seed, yea ppm seems low but this is what it can handle, last res change started at 400 ppm .5 scale 16 hrs later leafs were getting black ,so backed down to 350, drank down to 315,so did this res change 400 ppm, my system is DIY 5 gal buckets 3/4 pvc feed and drain 12 spray heads per bucket, spay nozzels are areosol type Like whats on paint spay cans , No they dont clogg, and the spray is def alot finer then what most use inthese diy apps. 15 gal res and 750 gpm lp pump.Thanks for answering

I try to keep my ph right at 6 only adjusting once per day. But as far as I can see from pics they look good, little saggy put keep doing what your doing ... maybe a foliar feeding would help ... just my 2 cents

This strain def like the lower PH 5.7 was was keeping 5.9 slower growth I was dealing with early on, Thanks I might try foliar feeding in a day or so ,I just cant understand what happend, hoping to get some insight on why, thanks Bro retired local 28 Tin knocker