The main reason why people say they have not volunteered is because they have NOT been ASKED.
So ask! Ask everyone you meet, do business with or see socially, from the owner of the corner shop to other parents at the school gate.
Have others in your office do the same and start asking people to become involved. There’s no need to be pushy, but a key part of recruitment success is to directly ask people to volunteer.
Let me know if it works for you.