update manager notifier stopped working

I am running Xubuntu 8.04 and until a few days ago the update process was working as it had been from the initial distro installation, ie. a red gear wheel appeared in the notification area advising me that updates were available. Clicking on the wheel started the update manager. Now a grey wheel appears and clicking on it only bring up a message saying that a package manager is working. Manually running the update manager after the grey gear wheel disappears confirms that the system had been updated less than an hour ago. It's as if update manager had been reconfigured to run automatically in the background, yet I had made no such change. Moreover, the "Only notify about available updates" option is checked on the Updates tab of the Software Resources window. Also, Update Notifier is checked on the Autostarted applications window and is present in the list of active processes in System Monitor. How can I get the notifier working again?