We've had quite a few working parties this year, which have - on the whole - been well attended. But you can't force people to attend; as you say, there are older folk, or those who can't commit to certain days.
Our Terms & Conditions state that members will, "Assist with working parties to help maintain and improve the site." Not all do, of course. And some members help in other ways - mowing the communal paths, or doing odd jobs outside of formal working parties. And there are always a few selfish ones....
I know what you're trying to achieve, but I feel a "softly-softly" approach is better. The members have got to feel that the site belongs to them and they want to work to improve it. They will put pressure on the few who won't make the effort.
And as a PS: Have you looked at the insurance issue? We had public liability; we now have employers insurance as well, which specifically covers accidents to anyone "employed" by the Society - whether paid or not - and is designed to cover working parties.
And we find sticky buns passed round at the end quite a good idea!