It's really easy to shoot to the left, so many things can cause it. I think I've tried all of them at one time or another.
1. Keep your eye over the arrow. If the arrow seems to be flying to the target from the right to the left, your eye is to the left of the arrow. If your eye is over the arrow, the arrow will seem to fly straight toward the target.
2. Bend your bow arm slightly; at least, don't lock it out.
3. Don't torque the bow or bowstring. At full draw, if you relax the grip on the bow slightly, does the bow want to come to a more vertical position?
4. Relax your string hand and bow hand (see #3 above).
5. Pull all the way through the shot using your back muscles, so your string hand recoils back toward your neck when the arrow is released.
6. You can move the shot more to the right by canting the bow a little more, so long as this doesn't cause you to torque the string.
7. You can move the shot to the right by shooting a slightly weaker arrow shaft.
8. You can move the shot to the right by using a softer strike plate (a Martin Rug Rest or female velcro rather than a piece of leather).