I think the question is, is it FLYING nock left?
If not, it could be that the target material affecting the orientation at impact. If it is, then I would do as Arne suggests and really scrutinize my form/execution.
Personally, when I first started shooting trad, it took me quite a while before I realized that my form wasn't good enough to do any effective tuning. And it seems that that is quite a wide range of combinations for the result to be the same, unless it IS a form issue.
Just my $0.02.
Also, it would be helpful to the tuning gurus to state whether you are right or left handed.
Good luck, and I hope you get it figured out.