Persistent nock high is usually a result of form errors, but in some (rare) cases can result from shooting a bow 3 under that is tillered for split fingers. In either case, what is happening is that there is a true nock high when the nock is mounted high enough, which is why we recommend starting tuning at 3/4 or even 1". Then at some point the arrow starts rebounding off the shelf, which results in a false nock high.
You need to work your nock point down the string in very small increments, maybe 1/8" at a time. Start high enough that you see a reduction in the nock high as you work your way down the string. At some point, moving the nock point down does not result in any further reduction in nock high, and may even increase the nock high as you lower your nock point further. The point where lowering the nock point does not reduce the nock high is where you want to put your nock point, until you figure out how to eliminate the false nock high.
You can shoot accurately with a true nock high, which can be reliably corrected with fletching, but not with false nock high, which will result in inconsistent arrow flight.