your ph could be off. or you could be deficient in iron or nitrogen, possibly from pH nute lockout. i have been getting yellowing leaves that droop like straight down and fall off too. im in flower. i had been using a pool ph test kit, and i was reading it wrong or its i inacurate with drops, i thought my ph was like 6.7 its more like 7.25. they have probably been having trouble abosorbing iron on my end. check your ph
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with something reliable i got my ph pen on ebay for 29 with shipping included. WELL worth the investment. what nutes are you using?