My MIL was like this for years before showing any signs of dementia. We really couldn't understand it, she was exactly as the OP describes, agreeing to a visit or a lunch and then "not feeling well" at the last minute. Even her grandchildren's birthday parties, Mother's Day etc were spoiled. She would come to us for Christmas dinner, but was putting her coat on practically as the pudding was served! Holidays and day trips became out of the question.
We came to accept that we had to arrange things on the understanding that she probably wouldn't come - but this meant others (usually her husband) had to miss out too. It is upsetting, but looking back I'm sure it was all tied in with the onset of dementia. All you can do is be very patient, and just occassionally, she might surprise you and come out.
Her husband will understandably be frustrated by all this. Its difficult, I know, but if you can arrange somebody else to stay with Mum while he enjoys a meal out or a visit, it might help.
