I see, Ill try to research on dosing Potassium Nitrate before dosing it.Yup, thats what i meant by "nitrate locked". Often times, the absorption of N and P optimally stays within a fixed range of ratio in different organism, and experiments have shown that a system that has no phosphate will inhibit the absorption of nitrate in organisms like algae, (and vise versa). So since your nitrate is at 0 and high po4, its possible that the po4 absorption capacity of your different organisms are being inhibited by the lack of no3. Increasing no3 levels will "unlock" the po4 absorption in this case.
I haven't used potassium nitrate so can't really advise on it. I have used neonitrate before, with that product, I basically dosed daily according to the bottle instructions, and retested my params every 2 days. If nitrate lock was your issue, then you should start seeing po4 decreasing before seeing no3 increasing. This would especially be the case if you have algae (like a scrubber) as a part of your nutrient export.. With my tank, I saw an explosion of algae growth in my refugium and it was pretty effective and fast that brought both my no3 and po4 back to more acceptable levels.
So for your case, you dose it any time you have unstable N and P in your tank? Will the tank get to a point where I dont need to dose it anymore as the parameters are all always "unlocked" already?