Add-in Express RTD refresh issue

Question

Excel AddIn using Add-In express, VS2010, cometd.net talks to cometd server and Tom cat In RTDInitialize event handler, I log in Tom Cat and connect to cometd server, I use an instance of class credential to remember it is loggedOn(set IsLoggedOn to true) and sessionID.

In RefreshData handler of topic, I will register to cometd server if not register yet. Then I will subscribe topic to Tom Cat. Now when users delete the RTD function from Excel, DisconnectData handler is called, in it I disconnect from cometd server, logout from Tom Cat, clear out sessionID. Next time, when users enter RTD function again, it will call RTDInitialize event handler, login and connect where IsLoggoned set to true and sessionID is updated. The strange thing is after 2nd time login, in refreshData, I check the credentials, IsLogged becomes false and sessionID is empty just like what it was at first logout. Anyone know why? not sure if I make it clear. Thanks