What is "Three for me?"

We never stop trying. We use the Three for Me program to recruit volunteers starting the first day of school. The responses back are always high in number, about half of our school families at two schools. In the body of the introductory letter, I ask that they please note the dates and times of the volunteer opportunities and that the chairperson for those programs/events will be in contact with them closer to the time. I also send home an acknowledgment that their volunteer form was received and what they checked off.
The frustration comes when calling these people. I might have 20 people signed up for an event where four volunteers are needed and I can't get those four out of 20 calls made. Ninety percent of the time I get a voice mail and then I never get a call back. I don't understand that. I always return calls, usually within an hour of getting the message.
We do have some new parents jumping on board this year though. I've found a co-chair for our Cash for Trash program, a chairperson for our winter dance, Santa's Secret Shop and for our Schwan's Frozen Truck fundraiser. This happened because I directly asked these folks to do it. There is an advantage to directly talking with volunteers although it can be difficult to do if board members are not in the schools or activities where other parents, possible volunteers, are.

I was actually talking to a board member about this earlier today. We were discussing 2 different upcoming events, both with photographers, but one where the photographer was a volunteer the other an outside contractor. She felt the other photographer should be a volunteer too. While I agree I think we have set ourselves up so we only depend on a few, not many. A process we need to work toward changing.

So, long response to say...

I'll be watching this thread and suggestions with great interest!

Wondering if your group has fallen into the volunteer comfort zone (in the past or present)... in other words have you stopped trying to recruit new volunteers because it seems like you end up with the same group of people doing the work anyway? Like to hear how you have broken out of this comfort zone and energized your board to make the push to attract new members. Any tips or success stories you could share?