Consider a new mom's outing hosted by a long time member.
I don't know what your focus is, but we have associate organizers on our Meet Up group and the associates schedule events all over the county. There is no division, but people tend to attend what's close to them. Also, have things at different times and at different prices. I go to a museum with a kids' club and kid area that's only $4 admission once a month. I also do Meet Ups at parks, or libraries. If there is a local story time or kid event (wherever it is - Pottery Barn Kids, library, community center), someone who is already going should invite the group to come. That gives parents who are shy an event to focus on that is not heavy-duty interaction.
There will ALWAYS be those that sign up and never attend. And there will be those that only attend certain events. But try to mix it up a little and encourage others to suggest events or host their own for the group.
We also have Mom's Night out once every couple of months, where we ditch the sneakers and go out for the night as a group for dinner or drinks or an art exhibit. Ask them what prevents them from attending the MNO and see if maybe a lunch date would be easier for some of them or coffee on a Saturday afternoon.
The Meet Up format also allows us to easily schedule, reschedule RSVP, see the calendar and communicate with members so if your current format doesn't allow for a lot of chatter (we have a discussion board, too) between people, and easy access to event info, consider changing it up. If you want an inexpensive website, consider Weebly. You can use Google for your calendar and Nabble for your forum.