Taz, I think you solved your own problem. Sounds like your are watching your arrow instead of the aim point. Pick a tuft of hair or some marking on the target then concentrate on that spot and release. All experienced bowhunters have had or have this problem come up sometime in this sport. Another form of trouble can arrise from dropping your arm. Target Panic. Practice holding your bow on the target until the arrow impacts. This will help overcome the problem.
Good Hunting