The mistake? Not reaching out to every parent who's expressed a willingness to help. If you sent out a volunteer form in August, I'd wager that many parents expressed at least a general willingness to help. Where are those forms now?
Yes, it's way easier to call on your regular, reliable volunteers. But it's also a surefire recipe for sending a "we don't really want help" message to the rest of your parents.
Make sure all your leaders and chairs make a point of at least reaching out to new potential volunteers for each event. The newbies may not step up, but your asking sends a powerful message.
We have plenty of resources on this topic: