Cancelations happen all the time, so you should keep trying. If you are going around Christmas, you should continue to try daily. Many people book ADRs around Christmas "hoping" that they will be able to go to WDW, but once they find out that they can't get the air fare they were looking for, or that work is just too busy, they cancel. In other words, there are probably a fair number of holiday ADRs that are just "hedges" and will have to be cancelled as the dates get closer.