If you have a high drawing arm elbow, it can cause your forefinger to press down on the arrow nock, and bow the arrow downward slightly at full draw. The rebound from this can cause a persistent nock high that can't be eliminated by string nock placement. This can also be caused by not quite coming to full draw with good back tension, where your drawing forearm doesn't quite get in allignment with your arrow at full draw.
You can look in the mirror or have someone else observe whether your drawing arm forearm is in line with the arrow at full draw, both viewed from above and from the side. This will help your accuracy in general, not just the nock high problem. Another indication is whether your hand moves directly back and away from the arrow on release, as it should, or whether it moves to the side, as it will if you release before you come to full draw.