WebCT keeps track of which messages each user has read, and by default displays only messages which have not yet been read (these messages will have a unopened envelope icon to the left of the message in the message listing). Therefore, you will see only new messages listed in the topic frame - once you've read them, they will 'disappear'. You can view all of the messages (read and unread) by choosing All in the menu frame. (It will be replaced in the menu frame by Unread.)