Canada Post not forwarding mail

How can I fix this?

About a month ago I set up mail forwarding, from a PO box to my home address.

Exactly one piece of mail sent after that was forwarded to my home. Everything else has gone to the PO box.

It's all lettermail, all the mail has been addressed identically including the one piece that was forwarded, and nothing is marked "do not forward". Most of the non-fowarded mail was sent after the one forwarded piece, so I know mail forwarding has taken effect.