It's one of two things:

1) Your nock set is defined by two areas of serving material on your string (My preferred method). If this is the case....you're going to need to carefully remove some of the serving material to allow your nocks to "float" freely between your two nocking points (servings).

or

2) Your nock set is defined by your D-loop spacing (top to bottom). If this is the case.....the simplest thing to do would be to remove it and tie on a new one. You can do this yourself, if you have the inclination and the know-how, or have someone do it at a shop. What "I" would do is have them serve a small nock set above and below your nocking point that wouldn't allow your D-loop to slide on your string and "pinch" your nock.

Good luck.