When you emailed the parents asking for feedback, did you offer specific options for them to respond to, or did you leave it open-ended? It can be very difficult to send constructive feedback, especially when you're feeling like things won't change no matter what you do, if the question is open-ended. It can be hard to feel like you're able to say something constructive without getting too negative.
On the other hand, if you can present something closer to a survey, with potential shortcomings acknowledged upfront and potential solutions offered as options, you might get a better response. So, for example, using some of the issues magiccat has brought up, instead of saying, "What can we do better?" you could ask for very specific feedback on very specific issues. Such as,
1) Meeting frequency
Would you be more or less likely to be able to make time to come to a meeting if they were less frequent? If we had meetings every other month, or quarterly, would it be easier to make it a priority? If we offered an alternate time for the same meeting content, would that help you be able to schedule time to come?
2) Meeting length & content
Do you feel like the meetings have enough useful content for you? Would you prefer more "meat" and less ice-breaker type filler, or do you enjoy the chance to visit with other leaders? Of the meeting components a, b, c, and d (where those are the things you do in a regular meeting - ice breakers, minutes, event planning, etc.), which would you prefer to minimize or eliminate altogether?
3) Email distribution
Do you feel like there is content addressed in the meetings that would be better served as an email announcement? Do you feel like you already receive too many emails from us?
Anyway, those are just some samples of more specific questions you could ask that might prompt some who do have issues to be able to respond without feeling like their problem is too big to know where to begin, or like they'd be getting too critical if they were to address the things they've been thinking about.