When you flushed your plants, how much water did you use? Did you just switch to giving them plain water instead of nutes or did you actually flush the pots well? Im no expert but if it were me, Id flush with about 2x the volume in water as there is soil in the pots initially, so for a 5 gallon pot, Id flush with 10 gallons of water to try to get a lot of the salts out of the soil. Also I think it would be better to not dump it all through at once, maybe half and wait an hour and run the other half through...