I think technique has a lot to do with it, too. I usually shoot recurves, and have never had that problem with one. But I also shoot longbows, and have had the problem with several of them. Since I can shoot a series of arrows that are well tuned for the bow (i.e., bare shafts group with the fletched arrows) without getting the slap, and then start getting it with the same arrows, I know it has something to do with my technique. It isn't obvious, though, or I would just stop doing it. I think I just need a really good, smooth release, and not press down on the nock with my forefinger, to avoid the slap. But there have been times I've thought I was doing that, and still had the slap, so it must not take much of an error to cause it.