If you know your spine is not stiff it might be you are not getting the nock under your eye. I run into this from time to time. When I anchor properly I really jam my thumb hook into my jaw and get solid bone to bone contact between the both. If I get lazy and don't do this step my nock sits out to the right of my eye and I get left flyers.
Not an expert in the least but, anyone wishing helping you will want to know the bow maker, poundage, arrow spine/ length, tip weight, draw length ect. If you shooting off the shelf or a rest. I am new to traditional archery and the ppl on this site have helped me a lot. Very good advice on here.
make sure the string blur is centered over your eye
every now and then this happens to me. come to anchor notice where the string blur is and correct it to be in the center of your eye and centered to your arrow.
jeremy, let's just assume, for a minute, that your equipment, ALL of your equipment, is perfect. I have recently experienced a similar phenomena, I think. (I'll bet your shooting at twenty yards, or so, too, and when you are really close to your target, you hit closer, but just a tad left. I am also betting that you are right handed.)
I have been working hard at coming back from a "stupid, non-archery related" shoulder injury. I have had to drop bow weight I may not ever be able to get back. I also, pretty much, had to give up on longbows. I need more mass weight in the bow, now, and any kind of "recoil", hurts! So, I went back to a recurve, exclusively, as it seems a little more efficient than a longbow, at 45#--and at that weight, I need all the efficiency I can get, if I want to hunt.
I have narrowed my, "What in the heck is going on? Why am I shooting left?" issue to two things. 1. My grip. 2. Where my eyes are actually focused, at the release.
1. The grip. We are usually taught to have a pretty firm grip on self bows and old-school, straight handled longbows. With a recurve, we are taught to do the opposite. Well, with my recurve, as I transition away from the longbow, I have found that if I use a firmer grip on my recurve, my arrows don't hit to the left.
2. Point of Focus at the release. I don't want to confuse the issue with a discussion between "Instinctive and Anything else". What I want to convey is my observation that if I am actually focused on my arrow, or a "sight picture" of my bow, in relationship to my target, my arrows hit left. But, if, at the moment of release, I am completely focused on the actual target, my arrows are right on.
We are also assuming your form is consistent, your follow-through is consistent, etc.
Jeremy, there are some serious, outstanding, target archers on this forum. If they weigh in, listen to them. Although I've enjoyed some success in the hunting field and in 3-D tournaments, I am not a target archer, nor do I pretend to be one. I only spoke up because I seem to be having the same, or a simular issue, and I am adjusting from a low wrist grip, (longbows, self-bows), back to recurves, (medium to high wrist), and this stuff is absolutely working for me.
You can go out back, take a few shots, and very quickly, you will be able to tell if anything I said will be useful. If not, I suggest we both defer to the experts. On this forum, trust me, they are the real deal. Good luck.