Mike:
You say you don't think its spine, however you don't mention what spine you are shooting, only arrow length! Right handers shooting left is usually spine (too stiff). You can check the amount of center shot your self, simple hold the strung bow like you were going to shoot it. Now look at the limb and center the string on the limb. Without moving the bow shift you sight to the shelf and see where the strike plate is in regard to the string. Again (right hand), if the strike plate is left of the string its less than center, if its to the right of the string its more than center, if it centers the string its center cut.
Having said all that, Ryan is also correct you might be plucking the string, or moving your bow arm, concentrate on pushing the bow to the target and when releasing the string coming straight back from your anchor.
Bob