This has happened every year to my son. The child he thinks of as his best friend does not invite him. I have suggested that maybe they are doing something he won't like or maybe its something he knew other people would like better or maybe he was feeling a bit grumpy at ds the day he issued the invites. There is no easy answer. To be fair though, ds has always seemed less bothered by this than me and has a variety of explanations of his own which seem to satsify him.