This really is a big problem for host families when the au pair doesn't turn up on arrival day, giving only poor excuses or sometimes not even getting in contact at all. Au pairs need to look at this from the family's point of view: they've taken out insurance for the au pair, rearranged their working hours, and perhaps even taken holiday to accomodate the au pair's needs. And then the au pair doesn't turn up and they have to start their search all over again.
Our message to everyone in this situation is to stick to the agreement you have with your au pair/host family. If something changes let them know, if not then carry out the au pair stay as agreed. It is better to make sure in advance that the host family is the right one for you, and to either invest in a trip to get to know each other, or skype with them, before agreeing to anything. However, if the parties realise during the au pair stay that it really isn't working out between the au pair and host family, then steps can still be taken to make a change. And if pulling out really is unavoidable, then please get in touch with the other party right away, and not the day before, or worse, on the planned day of arrival.