“And he (one of the 3 angels) said, ‘I will surely return to you at this time next year, and behold Sarah your wife will have a son.'” (Genesis 18:10)
Of course, Sarah did have a son – but scripture doesn’t record anything about this angel returning a year later.
The phrase “ka’et chaya” is usually translated as ‘at this time next year’ – but the Hebrew is not so clear. A more literal reading would be something like ‘at a time of life’.
Rashi, in Sefer HaPardes, suggests that this angel did return to preserve the life of this son who would be born. Many years later, when Abraham was about to sacrifice Isaac, it was this angel who called out to stop him. It was through the intervention of this angel, years later, that Abraham and Sarah did have a son who would carry on their mission.
Often in life, we don’t feel that our prayers are being answered. We have to realize that sometimes, the answer might come years later.