Some chatboxes don't have "Seen at #:##." Help?

Hi there r/facebook,

When I send a chat message to someone, and I know for a fact that he/she read it, sometimes the message "Seen at 11:19 pm" won't show up. Does anyone know the reason for this? Is it just a small glitch? How did the recipient manage to view the message but not have the "Seen at #:##" visible to me?

Thank you!