Plant stopped eating nutes. Please HELP, Week 9

Hello all, I'm still fairly new at this and ran into a big problem... PLEASE HELP!! I'm in week 9 of a hydro grow and everything was doing good till just after my last water change. My plant seemed to stop eating, growing and the PH keeps dropping from 5.8 to 4.6 in just a few hours. I haven't had to add any nutes in almost a week now and the PH keeps dropping so fast I can hardly keep up with it.
I hope someone might have some ideas on what to do. So far the plant still looks.

sounds like there done. but to identify your problem we need pics.

It could possibly be a root rot issue or salt build up from too much nutes and no flushing.