OK - I think I understand. Eventually, they do all rest to Zero, but it takes longer than usual. The fact that is does serve a ball suggests that there could be an issue with the switchstacks on the score reels. There is a set of contacts that closes when the score reel reaches "9". I don't have a picture in front of me, but it might be on the "10"s reel, and flowing to the others. Clean or gap that set of switches on the "10"'s unit as a start.
