Also remember that at full draw, there won't be as much space between the double nocks as there is when the bow is not drawn. If you leave the right gap between your nocks, the arrow nock will be snug at full draw, even though you can wiggle the arrow between the nocks when the bow is not drawn. In fact, if you make the nocks too close together, so the arrow is snug between them when you nock the arrow, the pressure at full draw will gradually move the nocks apart. This could be why your gap is increasing. It took me a while to figure this out, but I eventually noticed that the serving was spreading out between the nocks because the arrow was forcing them apart at full draw.