What does it sound like when he shoots? If the brace height is too low, nocks too tight, or there's plucking of the string during release, the arrow can 'slap' the shelf making a really noisy shot. The nock will be seen making erratic moves as the fletching tries to get control of the arrow.
If the arrow is hitting the shelf, you should also be able to see signs of damage where the fletch is scraping the rest.