I can only suggest that you don't give up and focus on the ones that have already made plans to attend. They will go back and tell others who didn't attend what they missed. Sometimes word of mouth is your best guided source.
I think that getting family members to reply and to pay their dues by the deadline has to be one of the hardest part in planning the family reunion. We always have too many last minute people wanting to jump in, thinking that it won't be too big of a deal. That's where the problems and hurt feeling generally begin. They fail to realize that caterers and other bills need to be paid before the reunion takes place. I expected hosting a family reunion would be time consuming, but not to the fact that you have to continuously have to remind "grown" people about their obligations. That's very stressful.
We are currently experiencing the same thing too. We started last year with the planning of the reunion. We sent out four different letters concerning the family reunion and giving deadlines for reservations and dues to be taken care of. We even have the family website up and running to help keep family members informed. We have gotten some responses, but we have heard that more are planning on attending, but we haven't received any dues. It has placed a lot of stress on us. This was our first time hosting the reunion and we look forward to having a good time. We just continue to pray that things will progress and things will get better. Family reunions are great, but if you don't get the cooperation and support needed, it could turn out to be a nightmare. We have about a month before this event occur and we are going to have positive thoughts and do the best that we can.
Hope things work out for you.