lil brown spots

before u ask yes i have looked thru the forum for other post like this i even when through a plant problem sticky that had a lot of good info just nothing that helped

any ways i have lil dots starting around the veins in the leaves then it spreads and kills that leaf... wtf

i have check my ph its at 6.1 6.3 for my flowering plants and it happens to my plants after they are in flowering for about 2 weeks they start to show sign of this
i use fox farm nutes and i have just added camg+ by general hydroponics to see if that help but a new ph test kit with plenty of ph up and down but i dont think ph is the problem i think it might be lock out
here some pics take a look and plz help

ok it could be a ph problem i checked my run off and it was at like 4.9 5.1 area
but if yall think it could be anything else let me know i ma open ears