I can't tell from the pic for sure but it looks like that's where your arrow is riding ,knock a arrow and see where it's touching , maybe lower the knock and see ,I have been tuning arrows and shelf heights & location a lot lately , seems there is always a sweet spot on every bow I have , the hard part is finding it ! But when you do it's worth it , some times I make shims out of painters tape to see where the arrow touches & height and don't have to change the string knock tell I find the recipe ,I hope that helps ! Some times bare shaft tuning doesn't tell the whole story for me.