This motif recurs throughout the novel; people get incredibly close but no cigar. The timing's just off. Some examples:
Oskar's grandfather visits his true love Anna's house five times and never finds her. He's about to give up, but finally runs into her. She's never home because she's been going to his house.
Abby Black calls Oskar soon after he leaves her apartment but she misses him. He doesn't get the message for eight months because he's afraid of the phone and answering machine.
William Black actually meets Oskar's father, but doesn't know that the key's inside the vase he sells to him because he avoids reading the letter in which his father told him about it.
William makes a sign to post looking for the buyer of the vase but there were posters everywhere on 9/11, including one of Oskar's father.
Oskar visits William and Abby's house. William, who knows about the key, is in the next room but Oskar doesn't meet him.
One of the Blacks Oskar tries to visit was killed on 9/11. Oskar finds out she was a waitress at a restaurant in the building where his Dad died. He thinks about how she might even have served him.
Oskar's Dad visits Europe to look for his father. He meets him, but neither reveals his identity and we don't know if either actually knew who the other man was.
Thomas, Sr. returns to the U.S. just days before his son is killed. He never gets to see him.
And worst of all, Oskar's Dad misses talking to his son in his last moments alive.
All these incredibly close coincidences and near-misses seem to reflect the complicated, random nature of human relationships. We're all just wandering around, desperate to connect and not always succeeding.