At the wedding of the child of a friend of my mother's, 20% of the Yes RSVP's didn't show. At my wedding it was closer to 50%. :|
That really stinks! What did these people say to you when you saw them later?
The only one who gave a 'real' answer (i.e. this is what actually happened, rather than a white lie to make us feel better), was my brother-in-law, who finally admitted that his now-ex wife had picked a fight with him that morning, so she could refuse to go (she admitted her motivations later). And refuse to let the kids go.
My DH has not spoken to her since. I don't much blame him.