why is it difficult to explain for biff to willy what happened in bill oliver's office?

Biff waits all day for Mr. Oliver to return. When he finally shows up, he does not remember Biff at all or even that Willy had once worked there. Feeling dejected, Biff steals a fountain pen from Mr. Oliver's desk. Biff is humiliated by his father and by the fact that he has no prospects. He refuses to talk about what happened again.