"New posts" icon isn't being updated!

[Jujubee]

Here, and on other forums, when I view a "new" thread and then go back to the threadlist, the new/hot/old thread icon changes to the "No New Posts".

But on my board the "New posts" icon remains! I can view the thread 20 times and force a refresh on the thread page and threadlist pages, but the icon stays at "New posts".

It only reverts to "No New Posts" when I leave the board and return 15 minutes later (that's what my cookie timeout is set to).

Why is it doing this? I looked through forumdisplay.php and it seems cosher:

PHP Code:

  if ($bbuserinfo[lastvisitdate]=='Never') {
        $thread[newoldhot]='new'.$thread[newoldhot];
      } elseif ($thread[lastpost]>$bbuserinfo[lastvisit]) {
        if (!isset($bbthreadview) or $bbthreadview[$thread[threadid]]<$thread[lastpost]) {
          $thread[newoldhot]='new'.$thread[newoldhot];
        }
          eval("\$thread[gotonew] = \"".gettemplate('forumdisplay_gotonew')."\";");
      }