It's hard to tell from the video (even when I slow it down), but it kind of looks like it's not always returning to the resting position when the solenoid releases. It's possible that there's a problem with the spring or it could just be getting hung up. it's possible that the plunger is mushroomed or that the sleeve needs to be cleaned or replaced.
What happens during the reset cycle? Does it ever get hung up at that point or just when scoring 500?