Please help me. plant still suffering

I've been having a prob with one of my plants of 3.

last week and a bit the plant has been suffering. not sure why. it get exactly the same as the other 2 plants but this one is looking much worse for wear.

can anyone give me any advice?

just changed it to qtr strength nute.

PH 5.5 coz run off was a touch high. 7.5

EC.05 - .06

another pic update. bottom seems to be really off looking and drying up.

alot more water in this plants res.

but new growth seems to be ok?