You don't say how many events and programs your PTO currently runs. I agree with a number of things in the responses already posted; most of all I think that it's a fact that it's time, work, and effort to cull and manage volunteers. Maybe it shouldn't be that way, but I think it usually is.
If there's a general and serious lack of participation, you might want to try for an all-out volunteer drive (which needs time, work, and effort!) As another person already posted, point out that parental involvement does a lot! The PTA website has a lot about how it fosters community which in turn helps behavior, cuts down on bullying, etc. Maybe ask people why they don't volunteer? Ask what kinds of projects they could get behind. That 'Three4Me' program that was posted in another thread seems to recognize some of the realities of getting parental involvement going. I say: Challenge people, and see what happens.
Also be mindful that a bad volunteer experience can turn a person off for good, while a good one can make them 'lifetime' volunteers. I have answered at least one 'volunteer call', only to show up and find there was nothing for me to do! Big waste of my time. I answered another, for an event that was some months off, and on the phone the person seemed quite pleased to have a new volunteer. Then I saw that the event came and went and she never called me! She probably lost my number, but to me it just says that I wasn't really needed anyway. Try not to let that happen. Much as I love to be in the school, I will not get involved with those people again, and if they had been my only experiences, well...
Someone recently said to me "the PTO isn't run like a business. I try to remember that." True, but it doesn't have to be run like a business to be run pretty well, and inclusively. It's volunteer, not forced labor. People who want to be in charge of something, but don't have voice mail or have email addresses that go nowhere: all bad. I'm not saying that it's someone's job to ensure that all parents should have loads of fun helping out, but neither should a parent show up as asked and walk out thinking "there's an hour I won't get back", lol. Honestly, it's hard to see how people could NOT want to volunteer once they see how it makes a difference to their kids.
My 2 cents.
Three4Me program website: http://www.three4me.com./
PTA website:
www.pta.org/aboutpta/index.asp