Well what I found with mine was that there would usually be 200 -300 ppm of pure fert sitting in the medium and I think it's causing reactions with the roots like nute lockouts and overfert despite the ppms of my solution being normal. (at say...1200)

For instance....too much phosporus (or something I can't remember exactly which) will lock out potassium and I kept coming up with k defs in my plants even though the ph was fine and I had tons of Bloom ferts available to the plant. Along with your ferts are the fert "salts" and I believe these cause the biggest problem......high salinity can and does cause lockout of several different nutes. I also found several times that along with several hundred ppms of ferts in my rinse the ph would be off....water going in at 5.6 and coming out at 6.2 for instance.....god only knows if that causes its own problems or not since 6.2 is still considered normal.....but for all I know the ph could be worse with build up than I am seeing in the runoff.

Also known as "toxic salt buildup" .