Uppp, I think you may have been reading my post too fast Arthur.
Bob, good practice on culling out the arrows, but you apparently don' t check an obvious and often totally overlooked source of trouble. Your nocks. Worn out, too tight, too loose, off center... all of them can cause an arrow to not group with others.
So don' t forget to change out nocks and test before trashing what might be a good arrow.
My post:
When this happens, I' ll usually change the insert (or simply heat and spin it at least 3/4 turn), and change the nock.
And I agree with you Arthur 100%. Those nocks can cause problems if they are not 100% perfect, especially with broadheads.