Why has my guestbook message not appeared?

Messages are checked before publication and are held for the host when a check cannot complete or when the host reviews everything.

A message goes through the same shape of check as a photo. If it passes, it is published — unless the host has switched on Require approval, in which case it waits for them.

A message can also wait for a human simply because the automated check could not reach a conclusion. The system is built to hold rather than to guess, so an unresolved check leaves the message unpublished and in front of the host rather than publishing it unchecked.

A private note never appears in the public guestbook at all — if that is what you wrote, it has arrived, and it is working as intended.