Datamanager doesn't update forum "last post"

[MoMan]

I use the following code in a script to move threads to a forum that is otherwise locked:

$threadman->set('title', "$customprefix $threadinfo[title]");
    $threadman->set('forumid', 81);
    $threadman->save();
        $threadman =& datamanager_init('Thread', $vbulletin, ERRTYPE_ARRAY, 'threadpost');
    $threadman->set_existing($threadinfo);
    $threadman->set('lastpost', TIMENOW);
    $threadman->save();

However, the "last post" column for that forum never updates unless I run 'rebuild forum information' through cron. What am I missing?

Thanks for any help!