I have never run RW but seen many do it well and many do it poorly. In the end its a concept that can happen in any media. Ill explain but its going to be long.
So a few things at play.
Salinity is the amount of dissolved salts (our fertilizers are in salt form this is different than sodium) in a liquid. Water naturally travels from low salinity to high salinity (osmotic pressure). So when the salinity gets to high the plants ability to uptake water is reduced and eventually stopped locking out the plants ability to take up nutrients.
Now even though your input levels are say 500ppm as the plant takes up water and salts at different rates you often see the ppm of the media rise. The smaller the container the faster this can happen. Then add evaporation to that which is pure water loss and leaves all the salts. So as the plant drinks and we get evaporation the salts become much more concentrated which leads to high salinity.
Further add to that the saturation of the media. As the media dries back the water become harder to pull from the media as the media will hold onto it stronger in a sense. Think of a soaking wet sponge then you ring it out as hard as you can and lots of water comes out but you cant get anymore water out of it while the sponge is still damp and still has water in it. And no more water uptake equals no more nutrients.
Now add to that the PH can change drastically because of these processes and that causes availability issues for nutrients.
Now those salts buildup in the media each time the dry back to much if your not getting enough runoff to remove the excess so the problem usually happens over time and it can take a significant amount if runnoff to being levels back down to where we need them.
Media has a CEC (cation exchange capacity) and although RW has a low CEC it can hold on to some of the nutrients so we use enzymes to basically free the salts bound by the CEC of the media so they can be flushed away.
After all is said and done you need a light feed so the plants dont go from a lockout to a deficiency so we use a light feed after to ensure this.
To correct it we feed more often with larger run off to prevent this from occurring again.
Thats the basics behind why i say what i say. You cannot just half assed it and if you want to correct it then it’s important to do what I suggested bit in the end thats your decision.
Flush with enzymes (i think recharge or
hygrozyme personally) using a ph and temp adjusted water and then feed a low ppm 400-600ppm nutrient solution that is temp and PH adjusted
Aqua