CM,
Sideways/sidewinder nock movement generally involves arrow spine. If they're still grouping down the middle for you, the spine probably isn't off far enough that the feathers, can't stabilize them enough to get them where you point them, unless you've made a correction for left or right impact?
You might try bare shafting a few of your arrows after shooting your fletched arrows, to see where they impact, relative to your fletched arrows.
A spine problem will become more apparent as you increase your yardage, i.e., start at 10 yds and move to 20 yds. Bare shafts impacting left of your fletched shafts indicate a stiff spine, while impacts to the right of your fletched arrows indicate a weak spine. (RH shooter)