It's strange.
I have two sites, both running joomla 2.5.8 and CB 1.8.1. In one site the new version notification and update build in joomla core has worked as expected. In the other site the notificaction dosen't appear, the new version is not discovered in Extension Manager and CB backend show me that I'm already using the latest version but it says 1.8.1 in both, installed and lates version, not 1.9.