I found the link about the nitrate dosing here:
Adding Nitrate to a planted tank
I've never personally added more than about 5ppm at a time (having had the information from the above link in the back of my mind).
Is it possible that the problem is a lack of potassium, and that when you dose high enough KNO3 then there's enough potassium in the tank, and that it's not a lack of nitrates?