What is the res temp? Do you smell a funky odor from the res? Get a pool thermometer (they are like $6.00) so you know what is going on with the temp.
Strip the bottom two layers leaves of flush with ph 6'5 with 1 ml pk 13/14 revitalise ya soil. Or add a small bit nitogen. I think u have contamination in soil or over nuiting them. Flush use straight water for nxt 2 feeds.