Quite a few comments in here made me chuckle because it's like there's a script. I worked with volunteers for a long time. My personal view is that shaming or threatening volunteers backfires massively.
In the school context, telling parents 'we need this and if you don't do it bad things will happen' is both threatening and just irritating. Don't assume people can drop their lives to fit in or that they will want to.
Phrases like 'it's always the same people'; 'no one cares' is annoying as hell and, frankly, implicitly smuggy-pants.
NOBODY is a completely altruistic volunteer. Even very very tired busy people who volunteer do so because they attach a personal reward to it: a benefit to their child, social kudos, warm fuzzy feelings... whatever. So don't masquerade as a saint. Nobody likes being implicitly told they are 'less helpful/committed/community-minded' etc than others.
I seriously believe that the simple words 'I don't want to' - no explanation / justification attached - is a valid reason to not volunteer and should respected. Not everyone gets a 'reward', whatever that is, for volunteering. And why should anyone do something they really really don't want to do just because others who got involved do so because it makes them feel good...?
I am a serial volunteer type. But I draw the line at engaging with our old parent council because their tactic of going around the room and asking each person in turn to say if they could help galled me - I will not be publicly put on the spot and shamed or judged. Result: parents stopped coming to meetings. No volunteers.
I do still take tasks on, but strictly on my own terms and always with the knowledge I might get no support and have to do everything myself. And I will never ever whinge or complain about it or guilt trip or manipulate people into helping. My commitment, my job. Odd though, I never really have that problem. I always end up with help.
There's an annual task which no one would ever do because it was tough to drum up support and very hard work. I took it on because I didn't want my kids to miss out (selfish, self interest...and I'm honest about that) and I always have enough support plus the teachers and staff behind me. I think it's because I appreciate any level of help, and don't demand more or complain about the others who don't help. I think people know they can do what they can, and they will never be talked about negatively for not doing enough. Those little bits of help all add up. And more people help me every year.
My 'reward': my kids benefit, I've grown to enjoy the camaraderie from parents I'd never ever know ordinarily... but also, I love the sour expression on the PC chair's face be because it's become one of the fun events on the calendar and I stay cheerful and relaxed about it. Winds her up so much.