this could be many things. first i'd see if my first few shots were dead on - and then they wandered. its possibe, and probably the most simple thing, that you are pulling the bow to the left upon release when a little tired. concentrate keeping your bow holding arm relaxed and not pulling it and don't grip your bow. check your rest to make sure all screws are tight. can't tell you how many times i've left a shop and the tech forgot to tighten things up. what kind of release do you have (namebrand and model). if its shooting dead on at first, and later wandering left, its probably you. but even if it isn't - i think that is the first thing to check.