I use a xmpp client to send announce message, such as Psi or Gajim. All the online users can receive the announce message, except the offline users. I do send the announce to all the users not online users.
The offline user can not receive the announce message when it is online again. But the offline user can receive offline message sent from other user.
I guess you are using external authentication or a custom authentication. In that case, ejabberd cannot access to the full user database and thus can only dispatch to known users, that are the ones that are online.
Alternatively, it is possible that you are sending an announce only to all online users.