As Wapiti said, if a coach observed you shooting, he would probably steer you in the right direction faster, but if that's not an option, here are a few, but by no means all, of the things that have caused me to be erratic in the past:
As maineac said, your focus could be shifting away from your spot. It is hard to believe, but sometimes we think we're focusing on the spot, but our focus has shifted to the arrow point or somewhere in space. One way to check this is to make a conscious effort to focus on your spot on the next shot after an erratic shot (more of a conscious effort than usual). If you hit your spot, then maybe you weren't really focusing on the spot when you shot the erratic shot.
Loss of back tension can cause erratic shots. The evidence of loss of back tension is creeping, where the arrow moves forward at full draw before you release it. The arrow should never move forward during the draw, and good evidence that you are maintaining back tension is if your string hand recoils backward and ends up on your neck or thereabouts. If your string hand flies out to the side, you probably either lost back tension or plucked the string.
Cupping your string hand can result in erratic shots. Your string hand should be flat and relaxed during the draw, and you should be able to feel it stretch out as you draw the bow. The only tension should be in the tips of your fingers as you form a hook to hold the string.