I too feel that having a really concrete goal in mind, which will benefit the children of most of the parents, is absolutely critical to getting 'goodwill' from parents.
A little while ago, the village (population under 3,000) I used to live in raised many tens of thousands of pounds pounds to build a new pre-school building over a very short period, while the neighbouring school struggled to get a few parent volunteers to the school fete. Why? Because the pre-school had always been very open to parents and there was already a warm relationship between parents and the pre-school. The need for the new building was clear and very tangible. All offers of help were met with a smile and with open arms. Activities and meetings were scheduled at times convenient for everyone - whether they had small children, were SAHPs or were working. People were asked to help, and thanked properly, not told to help and taken for granted.
If you have a concrete goal in mind, communicate it to everyone. Have a purely social occasion - a party, a quiz, a drinks evening, a fish and chip supper evening, whatever - where nobody will be asked to give any money (except to cover costs) and nobody will be asked to do anything on the spot. Share your fundraising ideas there. Ask openly for others. Accept criticism of past events or ideas, keep asking how they could be made better. Crack down hard on 'negative talk' ("nobody will want to do that, nobody cares in this town, just us"). If someone has a brilliant idea that everyone is keen to run with, people to help will appear out of the woodwork. Thank them. Do not force them to go on the committee. Don't throw your toys out of the pram and don't threaten, but do state calmly that actually, you do need some more help and if we are to raise x to do y, then you would really appreciate all the help you can get. Make jobs tiny and specific - 'who can do half an hour on the hook a duck?' will work much better than 'we need people to help on the stalls and we've got the whole afternoon to fill'. Only do what you can organise easily, cut your pragramme to the bone rather than trying to keep up events 'because they've always been done' and run yourself ragged.
It's really hard, because you can easily get stuck in the 'nobody's helping us' mindset which causes you to go round everywhere with a deep frown and the world on your shoulders ... and then nobody wants to volunteer to help because it doesn't look like any fun.