I wonder what size nocks you have such that your big serving isn't big enough. Maybe they are the problem. I have many sizes of serving starting at, I believe, 0.015 upwards to 0.025. One of them has always been enough for whatever string I am building. In a crunch I could double serve with whatever combo would make the nocks fit right. I have some crazy nocks on Arrow Dynamics shafts that have a very small end opening and are big in the main part. I just could not get a a happy medium, so I made the lower nock point a little bigger and closer to the upper nock point, which allows them to grip the nock (using a smaller serving size so the nock end fits just right. Some day I will figure out what nocks will fit these shafts and pitch the nocks that came with the shafts. For your problem, don't overlook using the upper and lower string nock points to help hold the actual nocks in place.